What George said. Ih the machine has never been vacuumed, open the case [with the machine unplugged for at least 10 minutes] and either blow or suck all of the dust out. Dust will cause overheating. Do not limit your efforts to heat sinks.

Also, install Glary utilities free edition. In combination with either Avast free or AVG it does what you probably need if there is a software problem.