Performance problems on EVGA 8800gtx

Ok i recently brought a brand new 8800gtx, installed it into my pc and ran 3dmark to compare the increased performance...

Running my previous 7900GS i had obtained comfortable scores of >5400 with a little tweaking of core and memory of the GPU.

With the GTX installed at stock speeds this dropped to 3000-4000.
I originally thought this would be driver related and rather than using the drivers EVGA website suggest (163.75) i dropped back to the ones the official nvidia site gives (163.71); no change, score were still around 4k ish.

Wondering if the PSU might be at fault i tried both changing the wiring and even switched in the older 480w PSU i had. Again no change, still way below expected benchmarks. Incidently the fps on world of warcraft had also dropped from 60ish on the old gpu to 10-20 on the gtx.

Riva tuner shows the GPU to be running at stock speeds on core, shader clock and memory clock. Have tried altering settings on the GTX with riva tuner and have had some success with this, although this doesnt provide any siginificant improvement and certainly doesnt bring the scores up to the 9k ish mark i should be expecting.

My system is as follows:-

Windows XP64 professional
Gigabyte DS3R motherboard
Pentium core 2 duo E6750 running at 3300Ghz o/c
4Gb geil black dragon pc6400 ram (2x2gb sticks)
Thermaltake toughpower 750w PSU
and ofc EVGA nvidia G80 8800GTX gpu

With the GTX installed i have also had an odd period when frequencies on the gpu core and memory dropped to below half stock speeds and have experienced 3 or 4 random blue-screen crashes, usually when running 3d applications.

My question i guess is what else could be wrong, what could i do about it. And i guess most importantly does anyone have suggestions for diagnostic tools or programs with which i could locate the problem.

EVGA have offered return or replacement under their RMA warrenty, but i would like to be sure before incurring the expense of shipping the card back to them and any help would be gratefully appriciated

Have done this and also tried 2 different power supplies.
Does nobody have any suggestions for diagnostics i can run to isolate the problem here?

This sounds like a card problem. Have you tried switching out the card with a new one to see if the problem goes away? That would confirm it.

I suppose since you seem to be so adamant about running something, you could use ATI Tool .27b (supposed to work in vista, dont know about 64-bit though) and run a scan for artifacts. Here's a link
http://www.softpedia.com/progDownload/ATITool-Download-8466.html

*I DO NOT TAKE RESPONSIBILITY FOR DAMAGE TO YOUR CARD, SHOULD THERE BE ANY.*