Maybe you could monitor your process list for a while:
ps -e

If the process IDs after a reboot differ a lot from the process IDs after a couple of minutes without starting/stopping anything (exept the ps process itself), this could be an indicator for something, that is restarting/crashing regularely.

Edit: And use "top". Press "c" after starting top, so it sorts your processes, that use most of the CPU.