I have had this problem for some time now and nothing has worked. Now I've got myself a new system except for my old 9800gt and the crashes still occur making it obvious that the gpu is the culprit.
The crashes occur usually after the intro video for a particular game ends and the menu screen comes up. Sometimes they occur after a few seconds of gameplay. They involve distortion of the game screen while the sound may keep coming out as normal. The screen freeze requires a system reboot but sometimes Ctrl-Alt-Deling and shutting down the process works too, if I'm lucky. This doesn't occur with all games and sometimes it occurs after minutes or even hours of gameplay.
Here's a list of games that I've experienced this crashing on:
Right on the menu screen:
After loading the game:
The Walking Dead Episode 3 (the first two episodes worked fine)
Skyrim (Skyrim worked fine a few months back)
Saints Row 2
A while after playing the game:
Dark Souls (crashes are rare but they occur sometimes while alt-tabing or during a cutscene)
I doubt it's because the game is demanding because many heavier games work fine. It can't be the temperature either because I tested Dishonored by running the game after keeping the pc switched off for a while.
My system specs: Corsair 650 TX V2 psu, 2500k, Windows 7 64bit(the crashes occur on the 32bit version too), Seagate 1TB HDD, 9800gt 512mb