I've had the same problem for about 4 years now, lol.
Ever since I had my PC, it would randomly 'restart' itself every once in a while. Some days its totally fine and it will run for weeks, other times it will 'restart'.
I just learned to live with it, I back-up everything. Funny thing is, my PC has NEVER gone down (to the point that I've lost everything, etc).
Strange, heh.