Original post by: Dan ,
If the system internally is clean and the fan is working you done the basics. OK, lets look deeper here. Did the system get wet at all drink spilled or sat on a wet surface? This could have been a while as well. If you don't think so then the next area is to look at the heat sink and the thermal paste joining them together thermally. Do a careful visual inspection of the heat transfer bar often these are filled with a coolant which does the work vs a solid metal bar. Do you see any corrosion or pin hole when something has leaked out? If so then the issue is poor heat transfer within the heat sink. If you don't see any thing the next thing is to take the heat sink off clean both the heat sink and the CPU chip of the old thermal paste and apply a fresh coat. I must warn you this is a big job taking off the heat sink. You do need to be very careful as you can't force things as you could damage the components or the heat sink. Make sure to use a good quality paste and don't over do it (more is not better).