Recognize these PC symptoms?

My main desktop PC, the one I play warcraft and occasionally code in, has been bizarrely unstable lately.  In a way that hasn’t been too easy to diagnose.  At first, it was only when I was in Windows, it would lock up or suddenly reboot.  Usually right in the middle of a melee.  Yesterday, it started happening when I booted into Windows, it was crashing/freezing at random times. 

Since its happened in both OSes, it must be a hardware problem, I’ve long suspected that the power supply cables are flaky because if they’re not in a the exact right position, the box might not boot up.  But this morning, after another crash, I reset the BIOS, turned off all the power management settings and left all the other settings at their default values.  The computer has operated without problem all day today while I worked from home.  

Any suggestions, short of replacing it with a Powermac?  I’m thinking of replacing the power supply, and if that doesn’t do the trick it might be time for a new motherboard.