lap top overheating / shut down

I have an Acer 5315 that seams to be overheating causing it to shut down.I have opened it up and cleaned lots of dust from cooling fan, the trouble is that the fan does not turn on before it shuts down. I have it on a cooling platform, but it does not help. Thing is if I turn it back on right away, the fan comes on, cools it down and then it is fine. Any ideas on what I need to do or try?