Help!!! Computer crashing after about an hour :(

Hey guys,
I am having an issue with my pc. I recently upgraded from a 19 inch widescreen monitor to a 24 inch. Its a Samsung T240HD. I cant play for more then an hour on my games ( COD 5, COD 6, Crysis) without it totally crashing. Everything locks up, and a weird color pattern occurs on my screen for about 5 seconds, which then changes to a black screen. I cant do anything so i must restart my pc. I had played about 6 hours Friday night on it with a Xbox 360 my friend brought over. Ran 100% fine.

I have the latest driver for my card....XFX GTX 260 Black edition. I also did a driver sweeper after uninstalling the old driver. I did not loose any fps, so i dont think its a too big of a monitor for my card. I play it with 1920X1200, 4XAA and everything on high and it runs as smooth as silk.

My dad suggested i bring down the resolution to see if that will do anything. After bringing it down to 1600X900 i was able to play a little longer, about an hour in a half until it crashed.

The card is refurbished.

When it crashed with 1600X900 the GPU temp was at 55C. Which i no for a fact should not make it crash.

Any help would be awesome.

Intel Core 2 Duo 3.00GHZ
4GB RAM
250GB HDD
ASUS Mobo
X-FI Fitality Xtreme Gamer Edition
GTX 260 Black edition
Windows 7 Ultimate 32 bit

Lots of fan cases so i dont think its a temp problem.