Problems with new builds

My son built a new system and I decided to tag along and we ordered two of each. Once they were both up and runing we both ran MemTest86 for 24 hours with no errors. He was able to run 3DMark06 in loop mode for 24 hours. On my machine the program would crash in loop mode but would run single test or repeats up to 11 times but crashed if set to run 12 times. Neither of us can make it through Prime95 torture tests for both sides of the core. We have exchanged video cards and he was able to run 3DMark for a couple of hours with my card but the program then crashed. I could not run 3DMark06 with his card in loop mode.

Thinking this could be a overheating problem we re-seated the heatsink with new thermal paste on my sytem but that did not help. All fans are working.

We have changed memory timings from 5-5-5-12 to 5-5-5-15 but that did not work. It could be a power supply problem but we don't have a different power supply large enough to test. We are of the conclusion that if we can not pass both of these test programs we have unstable machines and we will encounter problems down the way. Any suggestions?

Would appreicate any help.

Here are relative components:

Motherboard: MSI K9N SLI Platinum nForce 570 SLI Athlon 64/Athlon
CPU: AMD Athlon 64X2 4600 Dual Core
Memory: Corsair TWIN2X2048-6400 2GB DDR2-800 SMS2-6400 Xtreme Performance
Power: Antec NeoHE550
Graphics Card: EVGA e-GeForce 7900 GT O 256MB RoHS (256-P2_N584-AR)