System turning off and having to reset cmos? Help

Ok this is a brand new system build, Nothing is reused except the keyboard/mouse and 1 monitor and speakers..

i7 920 (stock speed)
- XIGMATEK Dark Knight-S1283V 120mm fan.
6 GB DDR3 1600 running at probebly 1066 cause thats what cpu set at.

Radeon 4870 1GB
Asus Deluxe v2 mobo
Corsair 750w PSU
WD Caviar Black 640GB HD
LG DVD-RW etc

Installed Windows 7 RC.

RealTemp app shows cpu temp at between 28-38 C at idle.

What the problem is, is that when Im NOT using the computer twice now ive left it running and been gone from 2-5 hours and then come back to the computer off, hit power and wont post. So reset the cmos. It starts up, tries to resume windows and fails, tried to load windows from normal and works.

Now this might be all my fault but i dont know. I read up alot about the OC ability of the 920 so I did a little testing, got it up to 3.66 and it seemed stable. Kinda high temp was in 80's but was working.. Left for 2 hours on idle, and it did the above. So got everything working again. But now it did the same thing at stock speeds.

Any idea's?

When I OC'd only thing I did was change the BLCK and Ratio, didnt touch voltage left it on auto. Did set my ram up to the correct speed tho.

Just fyi for anyone out there wondering. Apparently it was just the windows sleep mode. Turned that off, then set it to run prime95 while at work and it ran just fine for 6+ hours. Odd that it would screw with the bios as well tho.