Take the battery out - to eliminate it completely.
Not entirely true. If the Arctic Silver thermal paste (or similar) has broken down - it could result in a quick shut down.
Try taking the RAM modules out and running on one only.
If the problem persists - replace the active RAM with the other one.
You could try blasting it with a can of compressed air through the vents.
Dust can cause all sorts of probems.
If, after trying the above, you still have problems, as J-B has already suggested, a visit to the local tech seems to be the only other option.