Cleaning out the heat trapping dust is essential. Are the fans spinning? If so, you may need to look at a professional cleaning, unless you feel comfortable doing some deeper disassembly. Contrary to what some believe, TIM (thermal interface material) does not wear out or need regular replacing so replacing the TIM is not something I normally recommend UNLESS the cured bond between the heatsink and CPU has been broken. Then replacing the TIM is absolutely essential.