Random crashes/will not power on

I have been experiencing a very frustrating problem with my PC. For some time it went from random restarts or crashes where I could simply turn the PC back on to recover. It then started crashing and it would not power on from any power button (main button and power supply rear button) , the only way I could turn it back on was to unplug the power , hold down the power button for 20+ seconds and plug it back in. After that stopped working , the only way for me to power on was to remove/replace/reset the cmos.

I have no clue what is causing the problem , there are no error reports , all the crashes seem completely random taking anywhere from 5 seconds to 7 hours to crash, and the computer runs fine when it does work , which i believe excludes the power supply from the problem.

*The first time I replaced the cmos , the PC ran for a few months with no problems but the crashes returned.

*overheating seems to be unlikely because the computer is always in a very cool environment , but im not sure.




Windows 7 Home Premium 64-bit

Alienware
System Model: Aurora-R3

Processor: Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz (8 CPUs), ~3.4GHz
Memory: 8192MB RAM