Video signal dropping out in games

I've been trying to help my friend fix his system but I'm close to being out
of ideas, so I'm turning to you guys.

The system: Asus A7V600, Athlon XP 2500+ Barton, 512MB of DDR @ 333mhz,
Sapphire Radeon 9600 Pro, 120GB SATA Seagate, 400W generic P/S, extra 80mm
air exhaust fan, Windows XP Pro.

The problem: He'll be playing Wolfenstein Enemy Territory (Quake 3 engine) @
800x600, high-quality graphics (so not maxing the card or anything) and
everything will be fine for about 15-20 mins then *BANG* his video goes out
and his monitor reports NO SIGNAL. He'll reboot and start the game up
again. At that point, it starts crashing even faster (about 7 mins). The
system was brand new (custom built) in December and this has only started
happening in the last few weeks.

Stuff I've tried so far:
- install newest Sapphire/ATI drivers
- install newest DirectX
- tried the video card on my system (Athlon 1800+, A7V266-e, 512MB DDR),
worked perfectly
- ran MemTest86, memory checks out
- ran a voltage and temp monitor during gameplay, nothing seemed to change
right before a crash
- underclocked the CPU, still crashed at the same rate

What else should I try? Since the video card, memory, and P/S check out (I
think), is the Windows installation itself the problem?

Thanks for your help,
Mitchua