I started out having problems with my 8800 GT 512 MB pretty much as soon as I got it. I installed it at the same time as my new PSU that should have been sufficient for my system. Originally, my screen would go to black in the middle games, and the only way to resolve the issue would be to do a full restart. The issue got worse over time, and it eventually failed a year later.
Asus replaced it with a 9800 GT. Initially this was working just fine, but now I'm having (basically) the same error again, except my whole computer is turning off. I cannot turn my computer back on immediately after it happens, it usually takes about 15 seconds before I can turn it back on, and sometimes it'll turn off again on the second bootup. This sounds more like a PSU or CPU overheating problem from what I've read, but there's another issue...
In addition to the crashing, I'm having this weird flicker thing happening almost all the time. A horizontal row of pixels will jiggle a few times, and then settle back down. It's not extremely noticeable, until multiple rows start doing it at the same time. I thought it might be a refresh rate issue or something, but the monitor I got only runs at 60 Hz, so I can't really play around with it. I had a similar issue with my old 19" monitor anyway, so it seems to me like this isn't a monitor related issue. It seems to have gotten worse over time (more rows, more often), and it's more noticeable when there's more graphical effects on the screen, though that may just be because I'm paying more attention when I'm playing a game versus surfing the web. I am, however, able to find it almost any time I'm using the computer though.
I don't know what hardware might be to blame here (Graphic Card, PSU, etc.) If anyone has any thoughts, I'd be really keen to hear. I crash at least once a day now, and it's driving me crazy! A lot of games are now (though they weren't before) too intense for my system to handle, and I can't play through them.
Try to run the card on some other computer using same psu but different motherboard...If same problem occurs again then your psu is faulty....If everything works fine then your motherboard is to blame..