Check the coolant level. If it is okay you should have a coolant pressure system test done to see if there is a leak in the cooling system. If there is not you may just have some air in the system or a bad radiator cap.
Since there are other possibilites such as a bad thermostat, I am giving you a walk through diagnsotic for your situation. Just use the link below to get to it;
Saturday, November 3rd, 2012 AT 5:21 AM