It could be the CPU, probably the heat transfer stuff between the chip and the heat sink either dried out or gunked up or whatever and is not doing it's job, or the heat sink (assuming there is one) is gunked up. It could also be a fan that has dies, came unplugged, or is caked over in dust.

And of course, make sure the vents are clear too.

Are you overclocking it by any chance? That could be working against you too.