Crash after 1 hour

Hey,

Not sure if this is the right spot for this so apologies in advance if its not.

I just got my new PC and have found that it runs perfectly for about an hour then freezes. When this happens I cant see the mouse and keyboard inputs dont do anything.
If I then hit the reset button on the case it boots up to a screen giving safe mode, normal mode etc options for windows. I go into normal mode and relaunch any programs I had running and it works perfectly from then till I shut down, usually several hours later.

This has happened regardless of what porgram(s) I have running, and always seems to be about an hour after first boot up. I get no blue screen or anything, just a freeze.

I contacted the guy I bought it from and he wants me to:
Change the BIOS to boot frist from the CD drive.
Run somthing called "riva tuner software" to check graphics temps and increase fan speed.

I will try to do this tonight.

Im not sure if it is a temp problem or not because after restarting it runs without problem (even running Fallout 3 with everything at Max) and I would have thought that if it is a temp issue it would keep freezing after a time rather than just doing so once?

Anything else that I could try to fix this?

My specs are:
1: Q9550 (2.83ghz)
2: MSI P7N2 Diamond with Nvidia 709 Ultra Sli motherboard
3: Gigabyte Nvidia 280 GTX 1G video card
4: 8GB (4x2) Corsair XMS3 DDR3 1600MHz ram
5: 2x Segate 500 GB 32mb cache Hard drives
6: LG 20X DL DVD Dual Layer bruner
7: Antec 900 gaming case
8: Gigabyte Superb 720W Power Supply

Sorry for a long post but I figure it is best to try to be as accurate as I can be.

Thanks in advance for anyhelp anyone can give.
Mykl

Done.