I have been having this problem for quite some time and after searching through numerous forums and help sites, I just don't know what else to do.
The problem with my computer is that it turns off randomly and then restarts itself. Now, previously I figured this wasn't that big of a deal considering that it did not occur frequently. I have always left my computer on when not in use. Nowadays, it turns on and off continuously and can keep doing that indefinitely. Sometimes it will work for half an hour before repeating the cycle. Before I would just wait until it started working normally again and that would last for days without any problems. I always try to update the drivers when I can, clean it with air regularly, and the PC temperatures are always excellent. I can enter the BIOS without a problem.
Now, let me point out the parts I have replaced before I mention the parts it currently has. I have replaced the GPU, the power supply, and the RAM sticks.
Motherboard: EVGA X58 Classified
CPU: Intel® Core™ i7-950 Processor
GPU: EVGA GeForce 750 Ti
RAM: 4x Geil EVO Corsa 8GB
Power Supply: Thermaltake Toughpower 850W
OS: Windows 7 64-bit
Hard Disks: 800GB, 600GB, and 1SDD for OS