windows 7 - Computer freezes on cold boot

I have a problem which started about 3 weeks ago. Each time I boot my computer for the first time of the day, it will freeze around 1 hour after the boot. It happens independently of the OS in use (Linux / Windows). After a hard reboot, the system will be stable for as long as I don't close it. I've tested so far

1. checkdisk on the only hard drive of the computer, everything was fine




2. Testing individual memory sticks one at the time, still freezing




3. Unplugging every hardware pieces of the computer and clean/reseat them, still freezing




4. Updating or/and reinstalling drivers of a lot of stuff, still freezing




5. Stress testing the CPU and the GPU, system stable after 1 hour of stress test

I can also point out that it is not an overheating, nor an overclocking problem. I also don't have a spare PSU or a motherboard to test my system with. I'm looking for more tests or ideas so I can finally troubleshoot this problem.

The computer specs are

Dell XPS Studio 9100, 525W Dell PSU, Dell Motherboard, i7 930 @ 2.8 GHz, HD 7850 2GB OC edition, 6x2GB DDR3 RAM, Hitachi 1TB hard drive

Answer

Finally, after three months of system lock, I've found the issue. I've no magic answer, but resetting the CMOS did the trick. It rolled back the BIOS to the 2010 version and no more problem! It also started the first boot setup which may have fixed a few things.

Thank again to all contributors.