Some more details might help. Some devices seem to have known problems -- I had a first generation HTC Incredible, and on some Incredible handsets some of the motherboard components go bad after a while. At the end, whenever my phone would get hotter than a certain (low) temperature, it would spontaneously reboot. Then, because rebooting taxes the cpu and heats up the phone, it would throw my phone in a bootloop unless I was somewhere sufficiently cool. I've had many runs and bike rides ruined by this problem.
If you've noticed any trends in where, how, and what you're doing when it reboots, that might help diagnose your problem as well.