Just when I'd grown comfortable with the stability of my system (after about 6 months following the initial build), I'm experiencing problems that should not happen with my PC.

If I try to play a game of any type for any period of time, I will get a system reboot. It is highly annoying. So far this has happened in a few games, including Grand Theft Auto and recently Freedom Force.

I have:
Athlon XP 2100+
ATI Radeon AIW 9700
Western Digital 80GB SE HDD
1GB DDR Samsung RAM

The onl y other factor I can think of would be the PSU, but it's a 550W Antec, so that should be more than enough and the company seems reliable.

What I can't figure out is that my system should be capable of running these particular games at extremely high detail level, and do, in fact. But for some reason the lower the detail the longer I'm able to play. The crash is inevitable, but playtime is extended with lower settings. Normally, I'm able to keep my computer on 24/7 without such a crash (I've never seen one when not playing a game).

Also, it doesn't seem to matter what else is open. To test this I started the games when my normal amount of programs were loaded, and compared it to a fresh boot but with all non-essential processes eliminated. The result was the same.

Can anyone help me here? I just can't figure this one out...