Do you have good airflow in your case that is alot that can overheat and if the computer randomly shuts off ur problem is the processor is overheating, my frieds processor was to close to his psu and was running around 200 degrees feirenight.
Reboot problems are caused by the processor(In most cases or by mis cacutinig thins in the bios). My suggestion is to take a look at or processor and get an accurate temerature reading of your processor.
Post it here afterwards.
Also please tell me how close ur processor is to ur psu.