Check the t-stat to make sure it is working, if it is not, go to the dealer and buy a new one. If you can get the dual stage as mentioned in this thread. Do not get an aftermarket one, most are POS.
http://www.4crawler.com/4x4/CheapTricks/index.shtml#ThermostatIf the t-stat is ok, perform a leak down test on the cooling system.
