On a Power Mac G5, Dual 2.0 GHZ machine I have encountered the microprocessor fans taking off and running full speed when the tower has been in sleep mode. All control to bring the machine back is gone, no mouse control, no keyboard control, dark screen. The only exit strategy is to hold the power button until the computer shuts down and then restart it. Restarting it generally results in a functional computer. Once I did fail to have the screen illuminate and had no power on the mouse and the repair amounted to removing the lower CPU and switching it to the upper CPU slot. The apparently defective upper CPU produced problems when reinstalled in the lower CPU position and was removed which permitted the computer to operate satisfactorily. With the original upper CPU in the lower slot, the machine could not find the hard disk drive. Given that I do little work demanding of the dual processor mode, the upper CPU alone seems to do fine for me with no speed reduction observed. There was apparently a problem in the upper G5 CPU. A month after the repair, I am once again getting the mystery fan running during sleep mode and also got a CPU (0) failure report (Corrupt skip lists). The machine is still functional but seems to have a built-in boogie man lurking around. Any prior experience with this problem or comments would be much appreciated.