XFX 8800gt displays JUNK while rendering high graphics

my 8800gt displays junk while rendering graphics .. it runs smooth and i am able to move around windows easily .. but when i try to open a game , ( race driver grid , call of juarez , etc ) - it starts displaying junk(black screen with no.of paralel n vertical lines everywhere ) in my monitor . audio freaks out .. none of the keys work and the only way to bring it back is to restart . Now i am not able to play any games

P.S : my card works fine while operating windows( i am able to watch movies , listen to songs etc) . i am able to adjust all parameters in the nvidia control panel . But when it comes to games - all it displays is JUNK !!

PLS HELP .

system config
-------------

2GB DDR2 RAM transcend
320 HDD
intel DG33FB chipset
core 2 duo 2.66 GHZ
creative sound blaster 5.1VX
XFX 8800gt alpha dog edition DDR3
samsung 932n+ display

ya it worked very well until one day when the graphic card dint work at all (it kickstarts n stops) .. i gave it for service and they gave a replacement card . the replaced card had the aforementioned problem its weird !

done reinstalling drivers .

temps re normal

dint overclock

PSU - Zebronics PRO SERIES

if you download OCCT and do the stress test for the GPU and you get errors instantly and lots of them... then it is a hardware problem for sure. However, i am not exactly sure how you can convince them that its broken, unless u make it broken (sometiems the only solution when technical personnel is so unqualified, it took me 4 hours to convince a gateway level 3 support technician that an 8400gt is not a fair replacement for a 7950gt)...

so what do i do now ! .. its only 5 months old ... my heart s broken !

The good thing is it's still under warranty so you'll eventually get a replacement omce you find out if it's the PSU or or GPU causing the problem.