I would replace the fans with new ones (must be noisy and less efficient after six years of use), lift up the heatsink, clean it and apply fresh thermal paste over the processor then screw the heatsink tight. Once the top case is open it can help to dust off the internal parts with compressed air (dust can raise the internal case temperature specially if you're a smoker or have an animal at home). I would also take off the optical drive and do an internal cleaning using compressed air and isopropyl alcohol for the laser lens. It's like a car, you must open the hood once in a while to do a little cleaning, replace the spark plugs and the air filter