That's the thing is that it's hit and miss. Somedays, like today, I started up 3 or 4 times with no errors / blue screens / weird things. Unlike yesterday where I was up to 4 or 5 times rebooting before it was stable.
Take out any unnecessary hardware like sound cards and stuff and then run some basic hardware tests, memtest, seatools/wdtools and if you can't find anything wrong then stress test it overnight with prime 95 or something to try to recreate problems. After that I would suggest running an sfc /scannow repair to see if there are any corrupted system files and hopefully from there you have a good idea of what's going on.