Heres the dealio: I have a powerMac G5 tower (7.3) with PCI-X, AGP, Dual 2.0Ghz & 4x 512MB ram cards occupying 4 of the 8 ram slots (they are occupying the ram slots in pairs.) I am dual-booting OSX 10.4.11 and 10.5.8 on a 160GB sata hard drive. I also have an IDE drive with OSX 10.4.11 that I have been using to troubleshoot, but is not currently installed. Here is my problem: (short version) The machine boots when it wants to, often requiring several hard power downs to boot. (long detailed version) here are the steps to boot: 1. press pwr button, hear the click from the PSU. 2. Front fans briefly run full speed then immediately slow down to normal. 3. Next rear fans run full speed and immediately slow down to normal. 4. At this point in a successful boot, I would hear the apple startup sound, followed by audible hard disk activity (disk spinning, head movement), and finally the caps lock key will go through a series of turning on and off. 5. For an unsuccessful boot, the fans in the front will periodically speed up momentarily, then slow down there is no discernable pattern. No apple boot sound, and no hard disk activity. 6. Occasionally, when the unit is rebooting from a software update, and I am not present, I will come back and all fans are running constantly at full speed. 7. If the unit hangs, I press and hold the pwr button until I hear the click from the PSU and the unit powers down, back to step 1. When it boots, everything appears to work fine, and no indication of any issues whatsoever. Here are additional troubleshooting steps I have performed: Removed optical drive, replace with IDE hard disk with tiger installed, disconnected sata hard disk. Removed all drives, and disconnected USB keyboard and mouse. Booted from optical drive only with tiger retail dvd. All these attempts have resulted in the same symptoms. Finally, here are tools at my disposal to do further troubleshooting: Powermac G5 7.3 tech manual. Powermac G4 tower. C2D macbook External ide hard drive enclosure with several ide hard drives. 4 & 8 GB flash drives (the flash and external hdds wont boot) I dont have any external firewire drives, or spare sata drives for testing; and I dont have a firewire cable to boot any of my other machines into target disk mode. The unit is new to me, so I dont know its history. The previous owner is not helping, so there is a possibility it was problematic for him, and that is why he decided to sell it on craigslist. I plan on disassembling the entire unit this weekend and look for any distended capacitors or anything else out of the ordinary. I have never disassembled a Powermac G5, so I am apprehensive. I have disassembled G4s and g3s as well as limited disassembly of iBooks and g3 imacs. Sorry for the very long post, but I wanted as much information out there so you all can help me troubleshoot this thing.