Put a little fan under it, I swear to god it works. Like sit on your couch and put the laptop on your lap, then place a small fan under it. This worked for my dying laptop, which probably lasted a year longer due to me doing this method.
Are you sure it's the heatsink? It could be that the fan that channels the air out of the back of the laptop has stopped, has junk caught in it, or has failed. Get some compressed air and blow it moderately in the intake hole on the bottom of the Laptop that supplies air to the system. Just open up the laptop (Just like you are about to use it) and lay it on it's side so you can do this and also observe if any dust or debris is coming out as you blown it out.
This will have all the YouTube videos talking about your device, and how to safety take it apart if you are not a skilled tech. Be aware that heat can cause damage to other parts of the system so get a cooler to place under the unit to keep it cool until you can address the heatsink/fan problem. Look at the Cool Master NotePal: http://amzn.to/14UJGkU, this should help also.
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