Lots of times when I build a PC it will boot up ok but exhibit lock-ups later. I just finished repairing one I built about a year ago that locked up periodically. I swopped components out over time - memory, video card, hard drive, to no avail. I recently replaced the motherboard and that seems to have stopped the lock-ups. I know Mem86 seems to be the thing to use to test memory - is there anything similar to test a motherboard? or how do you test if the fault is in the motherboard?
Prime 95 is a free program that allows people to help discover new prime numbers. But more importantly it also does system stress testing.
It performs a series of higly cpu and memory intensive calculations, but then it checks the answers against known good results.
They mainly did this so people wouldn't run prime 95 on unstable systems and waste everyone's time, but it's now used more often for system testing than to help find prime numbers.
I keep it running while I am stress testing using other program, that way I know that not only has there been no crashes, but that my CPU performed a series of complex computations without a single error.