I had the exact same problem where my Asus 260 GTX would appear to artifact in games like WoW but by simply going on to the desktop and back into game would make it all better. I tested mine on L4D like you have, and it would also lock up and I would have to hold the power button to turn it off. At this point I was really annoyed and I was trying all sorts of things. I then found a PCI-E voltage setting in my Bios where I gave the card +0.3 more volts and the problem has not happened for 1 week.. even on L4D where it would crash in 5-10minutes..
So I would try to mod your card to allow some more volts since it has appeared to fix my problem.
So I would try to mod your card to allow some more volts since it has appeared to fix my problem.