It happened to me once - the same problems precisesly. It was on a 5-year computer that was extensively used - the silicon paste (I don`t know how it is named in english) between the processor and the radiator wore off and the cooling suffered baldy - CPU was coming to almost 70C, and it was the same - mouse froze, lockups, restarts...
Check Yours computer temperature - if it is something higher than 55-60C this could mean trouble.