Monitor randomly shuts off

Right now I'm using my 40inch tv for a monitor and have been for the past 3 years. While ingame I hear the disconnecting sound like when you unplug an ipod/peripheral. Then my tv turns off and back on. If i'm in a game like Mw2 it completely kicks me out of game. This happens like every 20 minutes too. If i'm in a game like counter-strike or blackops then it will just restart the monitor nothing will happen besides others say i lagg for a second. the graphics card im using right now is a Geforce 280 gtx and my power supply is a 700w toughpower by thermaltake. I was thinking maybe my graphics card was going dead because the same thing happened a couple months ago. I then switched ports on my graphics card and it stopped until recently. It's not my tv because it doesnt do this anytime except in a game, although it sometimes happens while just browsing the internet. The weird thing is this started happening when I upgraded my motherboard to a p8p67 Evo.