help please

have problem with comp after approx 45 min-60 min use comp freezes up or shuts down and only way to get it going again is to switch off power at wall as even reset switch and power button wont work any ideas ??? thanks in advance
 
Does the reset switch normally work? as I understand it, the reset switch is impossible for any software to disable, so if it's wired up and working normally, that ties it down to a 100% hardware issue.

It almost sounds like a thermal cutout in the power supply - is it in a cabinet with poor ventilation?
 
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