Could be caused by any number of reasons, overheating,bad memory,corrupt driver,unfortunately the list goes on. Does it happen after around the same amount of use? Do you get any error messages?
You could look in the event viewer after it happens and you should see a red circle which should have some sort of message which you can post here.
If it is caused by overheating then this will mean taking a side off the PC and with a soft brush and a vacuum cleaner make sure all the fans are clean including the one on top of the processor.
If it is ram or a driver issue hopefully the Event viewer will give us a clue.