Could be a number of problems causing this issue. Firstly the heatsink could be loose, or the thermal paste on the CPU has gone. Peering inside you would still be able to see the fan going, but nothing to actually keep the CPU cool, the freezing would be the user end result.
Alternatively it could be the HDD overheating or knackered.
My hunch is the CPU, if you can keep the system stable enough to install a program called SpeedFan (Google it) it will give you all the sensor readouts for ur machine (on the proviso it actually has your board supported.
Urban Kaos
January 2008