Do as Colin sayd, and also make sure that there is no obstruction on or near the vents. For example, if you work on a bed, the blanket will severly restrict the air flow...
The same hold true for any other kind of soft/cushiony surfaces.
make sure all vents are clear of the thick dust that normally accumilates there.
also if you are confident, also check that both fans are running & that the fan assembly over the processor is clipped down correctly.
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