Question : Problem: Intermittent shutdown!

I have almost a 3yr old system that I custom built and all, I replaced the PSU 5months ago and recently the system board since the AGP port is loose and does not show any video from my ATI video adapter. until now the computer acts weird and intermittently shuts down with the power led flea light still on and the and the keyboard numlock stays lit at the same time even though the power did shut down. I need to turn off the whole system from the UPS system to shut it totally down. I try to open it again and no problem I can work for more than an hour as if nothing did happen. I dont know what is causing it, could it be yet another bad PSU or a bad memory? Im eperience on repairing computers but this one bugs me. Any help?

Answer : Problem: Intermittent shutdown!

Random restarts and power downs is often a characteristic of bad memory or a bad CPU.  It could be the power supply, but a 500 watt supply is probably not being too stressed with your specs.  If you have already changed it, I'm betting that isn't the problem.  Try running a memtest and see if you can eliminate the memory as an issue.  These sporadic problems are hard to diagnose.