These are classic signs over over-heating. When my friend's PC had the same symptoms, the problem was quickly solved when I removed a bar towel from the top of the case. (It was hanging down at the back of the PC, obstructing he vents).
First check that there's nothing obstructing the air flow at the back of your PC. (It shouldn't be pushed too close to a wall). Then check that the fans are working. (You've obviously got one working OK, because you can hear it, but there might be another fan which isn't functioning properly). Otherwise, the problem could be linked to a processor which isn't properly mounted.
Chris