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The prime suspect here is dust on the heatsink - see if you can feel warm air from about 3-4" from your exhaust vent on the side. If you cannot, more than likely, a disassembly and physical cleaning is needed. This video shows what is involved in that http://www.youtube.com/watch?v=r4iDXWmpphk&feature=resu...
Another consideration is the surface the laptop is used on, it should be a flat hard surface (not a lap or bed) for proper airflow to the intake vents on the bottom.
Hopefully some of this will help.