Mac Pro randomly freezing

My Mac Pro OSX 10.6.8 is randomly freezing. It usually happens every 3-10 minutes. It usually, but not always happens when I access the dock, the screen will freeze, but the mouse will still work for another 5 - 10 seconds then both freeze for a few seconds and then all is okay. After doing this a few time then sometimes the screen will flash, just like when a pop up is blocked, then sometimes the screen will freeze light blue with short black lines all over it. Then it is frozen, but sound will continue to work. I can press power button and put it to sleep and wake it right back up and all is okay again. I did do a screen grab when screen was frozen, the image was of a normal screen. This has been doing this for about a week now. If I just turn it on and leave it alone it will be fine all day long. No problems. It seems some web pages cause this to happen more often. Any help would be appreciated.

It sounds like your graphics card is acting up. Do you have another card that you can swap in its place to test?

No, that is the only graphics card I have. Would it help to pull it out and re-seat it?

Yes do that and give it a good clean. If no joy keep your eyes out for an ATI Radeon 5870 Mac version card on eBay and such.

I replaced the original Nvidia with an ATI Radeon 4870 and all is good now.

Great news. A flashed 1GB card or a standard 500MB version? Only tricky part installing is getting dual leads connected.

It's the standard 500MB version. It was a challenge plugging those into the board.