hmm and this only happens after 20-25 minutes? Have you checked the temp of your CPU and Video Card when this happens? I doubt it but I wonder if something is over heating. other then try updating your video drivers or reinstalling them possibly.
Agree with Keithb1984, Update drivers & provide sufficient cooling - I had the exact same issue... yesterday believe it or not. This was happening on my 3D-CAD systems, I ended up cleaning the fan on the video card - and updated drivers for fun of it. Works fine, hasn't happened yet.