Monitor "No response" During Games

All of a sudden, whenever playing Call of Duty 2 my monitor shuts off and says "no response". The computer continues to run and I can still hear sound, but the only way to get the monitor on again is by restarting manually. I tried it with a cloned TV to see if the monitor was the problem, but the TV shut off as well. I've played CoD2 on here since Christmas and all of a sudden I get this problem. I unistalled the video drivers and reinstalled them without fixing the problem. My computer is AMD Athlon 64 2400, 200GB HD, 256 RAM, GeForce 6800GT 128MB, Acer monitor with 12ms response and 1280 x 1024 res 32 bit color. I've only had it for a little for 6 months, so it's not like something should be dying already. Any ideas on what it could be/what to do to fix it? Thanks a lot.