Cooling problem with 96 diesel

About a week ago my 96 1 ton dually started to over heat. the kicker is that it would only happen after a cold start. after i get 2 or 3 miles down the road the temp drops back to normal. I have replaced the thermostat 3 times, thermostat housing, water pump and done an infer-red heat test and can not find the problem. this is the first time i have had problems with anything on the truck. not bad for 478,000 mile. out of ideas