I have a Zotac GT240 1GB GDDR5 low-profile card. I have extreme problems with overlocking it. Especially on newer drivers.
The drivers constantly crashed and reverted to 2D profile. I went to an earlier version and it was fine. However, I recently started playing S.T.A.L.K.E.R Clear Sky and it keeps crashing(the drivers, not the game as you'd expect =P). The card is not overheating at ALL. I have a Coolermaster Elite 430 Black case. With a 140mm front fan, a 120mm side fan, an 80mm fan bottom, and 120mm fan rear.
What could the sudden problem be? It runs fine with Crysis and Metro 2033 even!
Right click on your desktop, go to NVIDIA Control Panel. Click on Manage 3D Settings and make sure you're looking at the Global tab. Scroll down to "Power management mode" and select "Prefer maximum performance". Restart your computer and then test STALKER.