I have a 2002 town and country LXi. Had a couple of the coolant lines go bad, had them replaced, also had the mechanic bypass the rear heater core because is started leaking at the same time the lines were replaced. but putt-ing around town, it is fine and We didn't catch it for a while because it rarely ever saw the highway,. As soon as you go over 50 mph it will almost overheat, we slow down, temp goes down. (right now you are saying replace the Tstat, I already did with a new one and still the same problem.) thought maybe a faulty t stat. took it out completely and still have the same problem. Bad water pump maybe? I think the problem was accelerated by the coolant, water mixture being off since we had to do a quick fill of water on the run and the temp this winter getting in the single digits before getting the proper coolant mixture in the engine. Mechanic said head gaskets were intact. Anything I am missing?

 

water pump,low coolant blown head gasket or warped heads.