Random crashing in 3D apps (Radeon 4850)

Hiya

Im having quite a frustrating time at the moment trying to diagnose my crashes.

System;

Intel Core2 Quad Q6600 OC'd @ 3ghz
MB = GA-EX38-DS4
4 gig RAM
2x Sapphire HD4850 in X-fire
WinXP 64bit


The problem started for me a few weeks ago. While playing Darfall my screen would just go blank, eventually reading "No signal" and the computer became unresponsive. After this happens several times I start getting random crashing outside the game and eventually the screen simple wouldn't respond after a reboot. I immeditaly assumed that my vid card was overheating ( at this point i was using a single Radeon 4870) but checking the catalyst control center revealed the GPU was well within working temp ( below 50dc). I figured it must of been my vid card in some fashion because the system itself wasn't crashing ( no .DMP created) and memtest reported no issues.
So, rather rashly, I just assumed the card was kaput. I picked up the 2x 4850 and everything seemed to be fine until a day after installing them the crashes started again.
After concluding that it probably wasn't the vid card ( same crashes with both card setups) I remembered that about 2 weeks earlier I installed a new PSU ( Antec ATX TruePower 750W Blue) to replace my old 650 watt which had a broken fan. This is the only component in my computer that has changed before the crashes started so I'm looking at it as a prime suspect ( defective?).

Few things to note;

- drivers are all updated...everything
- My CPU temp is fine. 38 idle and rarely getting over 45 at full load ( good fan).
- tried switching around the PSU cables in case there was some bad power coming through, no luck.
- I get crashes in other games not just Darkfall
- Have turned my CPU to default settings/tried running without Xfire...same problem.


I'm pretty much out of ideas. Contemplating returning the PSU or installing vista/win7 as my only other options.

Any help would be mucho mucho appreciated.


EDIT: Ok this thread seems to imply these crashes may be the result of new drivers or windows update. Sucks.

Exit out of catylist control center then unistall it. Use standalone drivers.
I had the exact problem.
Monitor turning black with yellow light, then a reboot?
Yeah its a nuisance.

I now use expert tool ati to monitor fan speeds and such.

See if this doesnt help you.
It did me