also the screen has frozen forcing a close and reboot
Sounds to me like a PSU problem , or most likely a Graphics card /drivers problem.Just possibly a RAM/Virtualmemory problem altho I would expect an error message for this.
Overheating of the Graphics card...or the Chipset(N Bridge) can cause freezes...check that the heatsinks are clean and that the fans where fitted are running- same time check and clean the CPU Heatsink and fan.
Update the Graphics card drivers(get latest set from makers website - uninstall old before installing new).
If the freezing persists - check the PSU voltages....you can do this with Motherboard Monitor (FREE Google for it- it will also show CPU and System temps)....all voltages (12V,5V and 3.3 should be within +/- 5% of the nominal values.
Often an unstable, overloaded or failing PSU can cause freezing.