Random restarts (graphics card related?)

I just built my first computer, and it had this issue ever since I put it together. I would be using it, everything normal, and the monitor read "no signal." I thought it was heat related, so I got a liquid cooler and replaced the thermal glue and got some nice case fans, one intake and one exhaust. My CPU runs at around 30C now, so I think I cleared that up.

The problem went away for a while. I played some games for a couple of hours with everything working fine. After a day or two though, the computer restarted again about 5 minutes after I had turned it on. I switched to a different older monitor to see if it was a monitor issue, but same thing happened about 5 minutes after startup.

I'm out of ideas, so I'm turning to the internet. I'm thinking it's a graphics card issue? It's not hot to the touch, but could there be some sort of connection problem?

Specs:
AMD Phenom II X6
ASUS Crosshair IV formula motherboard
EVGA GTX 560 Video Card (1024 mb GDDR5)
4 GB Corsair XMS3 RAM
Corsair H50 Liquid cooler

Are you overclocking the computer?

I'm too much of a noob to overclock haha. Now it's progressing to restart almost immediately after i turn it on (like 20 seconds)

Have you tried updating the graphics driver directly from Nvidia?