I will echo the recommendation to apply new thermal compound after removing the old. I will add however, that it is critical that you do NOT have any air bubbles in it. Air is a very good insulator and you don't want insulation here. You want good heat conduction to the heat sink. A little too much compound is better than not enough. Just don't go overboard with the amount. Spread a thin layer without any bubbles across the entire CPU surface. If it spills over the side, again don't go crazy, but it won't be a problem. This compound is a heat conductor not a conductor of electricity.
Dan