I haven't had this problem until recently. About a month or two ago my other graphic card died (It would only show random characters on the screen on startup). I then put in a replacement card, which the first port only shows hues of yellow. So I am using the second output of the card. It was working fine for the time being until I installed Windows 7. Now I don't believe its an operating system related issue. The problems first showed up when i would be playing games. I monitored the temperatures and I didnt notice a common temperature that it would shut off... but on just windows desktop with nothing running the Graphic Card runs at 70 Celsius. When playing games I have seen it go upto 89 Celsius. I then added an additional fan and those temperatures dropped 5 degrees. But more recently I would be just using word processing and surfing the internet and it would shut off. The Screen goes blank and music i am playing freezes... (the annoying sounds), but the computer stays on. What can I do to fix this?
try using on board graphic and see if the same problem persist. btw your idle temps seems a bit too high for that card.
other than that your psu may also caused the problem when your pc suddenly shut of due to unstable power supplied to your pc