When you say it's overheating, how do you know? Have you checked your temperatures with e.g. hwmonitor?
One possibility off the top of my head is that if your heatsink uses a heatpipe, which many or even most laptops do, the heatpipe may have become faulty - most likely a hairline crack letting what's inside evaporate - meaning it won't function anywhere near properly any more. As I say, that is just a possibility, if you can give us some more information on the exact issuer you're experiencing then we'll know whether that's applicable to your issue or not.