This problem started a day or two ago.
When running a 3D application, the screen will go black and say 'No Signal', and the computer will lock up.
It started in a game. It loaded just fine and I was able to play for a while, then it suddenly locked up. I restarted the computer and tried again, and it locked up again after a couple minutes.
I tried installing the latest drivers, and it worked fine for a few hours, then locked up again.
I tried starting a different game, and it locked up while loading.
I then downloaded 3DMark and ran it, and it locked up again after a few seconds of the default test.
I have not had this problem before, and I have been using this system for about a year and a half. I am running Windows 7 64 bit.