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
either the graphic card is bad or your psu can't power up your pc properly when the card is present in your system. you can test you graphic card on other pc to make sure if the card is the problem.