Is this (the attached) likely to be a driver problem, or an overheating problem?

Basically (and this happens in HL2 as well), at some point in the game, these big sheets of colour start appearing -- they each look like extrusions of some surface already in the game, and they appear and disappear as you move around in the game.

It's an nVidia GeForce 7900 GTX. Driver version is 6.14.10.9147 (Forceware 91.47).

Immediately after quitting the game, the nVidia control panel showed the temperature as 68C, which doesn't strike me as that hot. I guess that it could be measuring GPU temperature, and the VRAM could be hotter, though.