Could it be overheating? (should have tried gas mark 4 ;-) )
Try an experiment or two - turn on laptop and leave it idle and see what happens then turn on (the laptop) and do some fairly intensive work - perhaps you can use one of the benchmarking software - and see what happens. If overheating then the lock-out should occur significantly sooner when the laptop is active as opposed to merely resting.
Regards,
Chris
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