Overheating would be most likely to cause a freeze, or a complete shutdown (as the Mobo orders it to when it detects high temperatures), but doesn't explain the spontaneous reboots. Software you recently installed can cause spontaneous reboots as I have got this before. If you have System Restore enabled (it really is a lifesaver than most people think), go back to the date before your PC spontaneously reboots, However you will lose all your installed applications you installed after that date, but from there you can easily track down what software caused it. Hope this helps