|Of the things you mention they are the least likely culprits.|
The most likely culprit is a failing power supply. This should be the first thing to check either with a known good power supply or a new one. The power supply is the easiest component to replace and the cheapest.
After that the RAM could be the problem. This can be checked out with Memtest 86. Download an run this, overnight if necessary and that will root out any memory problems.
The next thing to check is the hard disk. go to the manufacturers web site and download their disk diagnostic and run that. That will root out any problems with the hard disk.
It is extremely rare for a processor to fail in service unless it has been abused, usually by overheating.
Faulty driver don't normally cause a reboot, usually a blue screen of death. To eliminate that make sure you have your computer configured not to automatically restart on failure. Do that from System Properties > Start up and recovery and make sure Automatically Restart is un-clicked. Then you should get a BSOD which will give you an indication of what the problem is.