so to be sure that the previous kernel panics where not caused by my graphics card, I tested two other cards : Saphhire Radeon R9 270X from my Windows PC - worked well and was solid even while running unigene heaven ... after just a few minutes doing nothing (or just browsing the web) it crashed again... and a few hours back I decided to test the original Radeon HD 2600XT that came with my mac... and (of course) I got another KP ...
*** Panic Report ***
Machine-check capabilities: 0x0000000000000806
 family: 6 model: 15 stepping: 11 microcode: 188
 signature: 0x6fb
 Intel(R) Xeon(R) CPU           X5355  @ 2.66GHz
 6 error-reporting banks
Processor 0: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200001084200e0f
Processor 1: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200001080200e0f
Processor 2: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200000084200e0f
Processor 3: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200000080200e0f
Processor 4: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200001084200e0f
Processor 5: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200001080200e0f
Processor 6: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200000084200e0f
Processor 7: IA32_MCG_STATUS: 0x0000000000000005
 IA32_MC5_STATUS(0x415): 0xb200000080200e0f
panic(cpu 2 caller 0xffffff800301ae0a): "Machine Check at 0xffffff800301bfb2, registers:\n" "CR0: 0x000000008001003b, CR2: 0x000000014a64b000, CR3: 0x000000000647e000, CR4: 0x0000000000002660\n" "RAX: 0x000000000000095d, RBX: 0xffffff810526a000, RCX: 0xffffff81052746e0, RDX: 0xffffff800364d120\n" "RSP: 0xffffff810e893f30, RBP: 0xffffff810e893f40, RSI: 0x0000000000000000, RDI: 0xffffff8003629d60\n" "R8:  0x0000000000000000, R9:  0x0000000000000002, R10: 0x0000000000004650, R11: 0x00000000000249f0\n" "R12: 0x0000000000000000, R13: 0x0000069157e0097e, R14: 0x0000069157e00bda, R15: 0xffffff80036d49e8\n" "RFL: 0x0000000000000246, RIP: 0xffffff800301bfb2, CS:  0x0000000000000008, SS:  0x0000000000000010\n" "Error code: 0x0000000000000000\n"@/SourceCache/xnu/xnu-2782.10.73/osfmk/i386/trap_native.c:168
Backtrace (CPU 2), Frame : Return Address
0xffffff81051dde90 : 0xffffff8002f2fe41 
0xffffff81051ddf10 : 0xffffff800301ae0a 
0xffffff81051de070 : 0xffffff8003036f2f 
0xffffff810e893f40 : 0xffffff8002f46deb 
0xffffff810e893f90 : 0xffffff8002f47330 
0xffffff810e893fb0 : 0xffffff8003014dd7 
BSD process name corresponding to current thread: kernel_task
Boot args:  kext-dev-mode=1
Mac OS version:
14C1510
Kernel version:
Darwin Kernel Version 14.1.0: Thu Feb 26 19:26:47 PST 2015; root:xnu-2782.10.73~1/RELEASE_X86_64
Kernel UUID: 270413F7-3B44-3602-894F-AC0D392FCF8E
Kernel slide:     0x0000000002c00000
Kernel text base: 0xffffff8002e00000
__HIB  text base: 0xffffff8002d00000
System model name: MacPro2,1 (Mac-F4208DC8)
System uptime in nanoseconds: 7221325683176
last loaded kext at 3051127431155: com.apple.driver.AppleUSBCDC	4.2.2b5 (addr 0xffffff7f85d53000, size 20480)
last unloaded kext at 3144742392501: com.apple.driver.AppleUSBCDC	4.2.2b5 (addr 0xffffff7f85d53000, size 16384)
loaded kexts:
org.tgwbd.driver.NullCPUPowerManagement	1.0.0d2
com.apple.filesystems.msdosfs	1.10
com.apple.filesystems.smbfs	3.0.0
com.apple.driver.AppleHWSensor	1.9.5d0
com.apple.driver.AppleUpstreamUserClient	3.6.1
com.apple.driver.AppleMCCSControl	1.2.11
com.apple.kext.AMDFramebuffer	1.3.0
com.apple.ATIRadeonX2000	10.0.0
com.apple.driver.AppleBluetoothMultitouch	85.3
com.apple.kext.AMD2600Controller	1.3.0
com.apple.iokit.IOBluetoothSerialManager	4.3.2f6
com.apple.filesystems.autofs	3.0
com.apple.driver.AppleOSXWatchdog	1
com.apple.driver.AppleHDA	269.25
com.apple.iokit.IOUserEthernet	1.0.1
com.apple.Dont_Steal_Mac_OS_X	7.0.0
com.apple.driver.AppleHWAccess	1
com.apple.driver.AudioAUUC	1.70
com.apple.driver.AppleMCEDriver	1.1.9
com.apple.driver.AppleLPC	1.7.3
com.apple.driver.AppleHV	1
com.apple.driver.ACPI_SMC_PlatformPlugin	1.0.0
com.apple.iokit.SCSITaskUserClient	3.7.3
com.apple.driver.XsanFilter	404
com.apple.AppleFSCompression.AppleFSCompressionTypeDataless	1.0.0d1
com.apple.AppleFSCompression.AppleFSCompressionTypeZlib	1.0.0d1
com.apple.BootCache	35
com.apple.iokit.IOAHCIBlockStorage	2.7.0
com.apple.driver.AppleIntel8254XEthernet	3.1.4b1
com.apple.driver.AppleFWOHCI	5.5.2
com.apple.driver.AppleUSBHub	705.4.2
com.apple.driver.AppleAHCIPort	3.1.0
com.apple.driver.AirPort.Atheros40	700.74.5
com.apple.driver.AppleIntelPIIXATA	2.5.1
com.apple.driver.AppleUSBEHCI	705.4.14
com.apple.driver.AppleUSBUHCI	656.4.1
com.apple.driver.AppleACPIButtons	3.1
com.apple.driver.AppleRTC	2.0
com.apple.driver.AppleHPET	1.8
com.apple.driver.AppleSMBIOS	2.1
com.apple.driver.AppleACPIEC	3.1
com.apple.driver.AppleAPIC	1.7
com.apple.nke.applicationfirewall	161
com.apple.security.quarantine	3
com.apple.security.TMSafetyNet	8
com.apple.driver.AppleIntelCPUPowerManagement	218.0.0
com.apple.iokit.IOSCSIBlockCommandsDevice	3.7.3
com.apple.iokit.IOUSBMassStorageClass	3.7.1
com.apple.driver.AppleHIDKeyboard	176.2
com.apple.driver.AppleSMBusController	1.0.13d1
com.apple.driver.IOBluetoothHIDDriver	4.3.2f6
com.apple.driver.AppleMultitouchDriver	262.33.1
com.apple.kext.AMDSupport	1.3.0
com.apple.AppleGraphicsDeviceControl	3.8.6
com.apple.iokit.IOSerialFamily	11
com.apple.kext.triggers	1.0
com.apple.driver.DspFuncLib	269.25
com.apple.kext.OSvKernDSPLib	1.15
com.apple.iokit.IONDRVSupport	2.4.1
com.apple.iokit.IOSurface	97
com.apple.iokit.IOBluetoothHostControllerUSBTransport	4.3.2f6
com.apple.iokit.IOBluetoothFamily	4.3.2f6
com.apple.iokit.IOFireWireIP	2.2.6
com.apple.driver.AppleHDAController	269.25
com.apple.iokit.IOHDAFamily	269.25
com.apple.iokit.IOUSBUserClient	705.4.0
com.apple.iokit.IOGraphicsFamily	2.4.1
com.apple.driver.AppleSMC	3.1.9
com.apple.driver.IOPlatformPluginLegacy	1.0.0
com.apple.driver.IOPlatformPluginFamily	5.8.1d38
com.apple.driver.AppleUSBAudio	295.23
com.apple.iokit.IOAudioFamily	203.3
com.apple.vecLib.kext	1.2.0
com.apple.iokit.IOUSBHIDDriver	705.4.0
com.apple.driver.AppleUSBComposite	705.4.9
com.apple.iokit.IOSCSIMultimediaCommandsDevice	3.7.3
com.apple.iokit.IOBDStorageFamily	1.7
com.apple.iokit.IODVDStorageFamily	1.7.1
com.apple.iokit.IOCDStorageFamily	1.7.1
com.apple.iokit.IOATAPIProtocolTransport	3.5.0
com.apple.iokit.IOSCSIArchitectureModelFamily	3.7.3
com.apple.iokit.IOFireWireFamily	4.5.6
com.apple.iokit.IOAHCIFamily	2.7.5
com.apple.iokit.IO80211Family	710.55
com.apple.iokit.IONetworkingFamily	3.2
com.apple.iokit.IOATAFamily	2.5.3
com.apple.iokit.IOUSBFamily	710.4.14
com.apple.driver.AppleEFINVRAM	2.0
com.apple.iokit.IOHIDFamily	2.0.0
com.apple.driver.AppleEFIRuntime	2.0
com.apple.iokit.IOSMBusFamily	1.1
com.apple.security.sandbox	300.0
com.apple.kext.AppleMatch	1.0.0d1
com.apple.driver.AppleKeyStore	2
com.apple.driver.AppleMobileFileIntegrity	1.0.5
com.apple.driver.AppleCredentialManager	1.0
com.apple.driver.DiskImages	396
com.apple.iokit.IOStorageFamily	2.0
com.apple.iokit.IOReportFamily	31
com.apple.driver.AppleFDEKeyStore	28.30
com.apple.driver.AppleACPIPlatform	3.1
com.apple.iokit.IOPCIFamily	2.9
com.apple.iokit.IOACPIFamily	1.4
com.apple.kec.corecrypto	1.0
com.apple.kec.Libm	1
com.apple.kec.pthread	1
Model: MacPro2,1, BootROM MP21.007F.B06, 8 processors, Quad-Core Intel Xeon, 2.66 GHz, 8 GB, SMC 1.7f10
Graphics: ATI Radeon HD 2600 XT, ATI Radeon HD 2600, PCIe, 256 MB
Memory Module: DIMM Riser A/DIMM 1, 2 GB, DDR2 FB-DIMM, 667 MHz, 0x8551, 0x373254323536393230484644335342202020
Memory Module: DIMM Riser A/DIMM 2, 2 GB, DDR2 FB-DIMM, 667 MHz, 0x8551, 0x373254323536343230454641335342322020
Memory Module: DIMM Riser B/DIMM 1, 2 GB, DDR2 FB-DIMM, 667 MHz, 0x80CE, 0x4D3339355435373530435A342D4345363120
Memory Module: DIMM Riser B/DIMM 2, 2 GB, DDR2 FB-DIMM, 667 MHz, 0x80CE, 0x4D3339355435373530435A342D4345363120
AirPort: spairport_wireless_card_type_airport_extreme (0x168C, 0x87), Atheros 5416: 4.0.74.0-P2P
Bluetooth: Version 4.3.2f6 15235, 3 services, 26 devices, 1 incoming serial ports
Network Service: Wi-Fi, AirPort, en3
PCI Card: ATI Radeon HD 2600, Display Controller, Slot-1
Serial ATA Device: KINGSTON SV300S37A60G, 60,02 GB
Serial ATA Device: WDC WD10EVVS-63M5B0, 1 TB
Parallel ATA Device: OPTIARC DVD RW AD-7170A
USB Device: Microsoft LifeCam VX-800
USB Device: DBT-122
USB Device: Wireless Receiver
FireWire Device: built-in_hub, Up to 800 Mb/sec
Thunderbolt Bus: 
Should I do a fresh installation of Yosemite, maybe using another installer (not SFOTT) ? or does anybody know another solution to this?