I have another theory...
The PSU is unstable. I need another power supply in there and the Compaq Evo's was only 220W max vs. ~300 on the MPC.
The 6600 would be welcome and I'm sorry if I didn't understand originally.
I wouldn't put the 7800 in the situation as it can draw 128 watts at idle and 244 under full load. That is why I put the 480 watt supply in the Asus computer you have.
The 220 should be ok for the motherboard an a hard drive.
Lastly your issues with the computer sound more like driver issues the power supply issues, but it doesn't hurt to ceck the voltages. Luckily all my motherboards measure voltages in th bios so I can check the voltages with the compter running.
I'll have a look I think there is some software that will give you a few voltage readings, install app below and run it to see wqhere your voltages are. Of course voltage without current doesn't mean your supply is good either, but if the voltages ae correct, probably safe to assume the current is there. If it wasn't you have a lot more problems.
http://www.cpuid.com/downloads/hwmonitor/1.25-setup.exe