Chrysler Repair: Heat lost while driving at higher speeds, radiator fans, radiator cap

Question
"I have a 99 sebring convertible and i have heat but when i get onto the freeway i loose all heat and turns cold, we have change thermostat checked hoses burped the system and changed the radiator cap there is no leaks the heater core isn't pluged, can you help us,"


Answer
Hi Marsha,
How does the temperature gauge on the dash read when this happens? If it is in the normal range (1/3-3/4 scale) then I would expect that there is something wrong with the blend-air door in the underdash unit which mixes the warm air with incoming cold air to produce the desired temp. Do you feel that something is responding when you move the temp lever?
If the gauge reads below 1/3 scale then I wonder if the radiator fans are running when they shouldn't be. You would need to check for that by opening the hood immediately after leaving the freeway to observe the fans.
Also, did you happen to notice the temp opening setting (F) of the new thermostat, either on the box or engraved on the thermostat itself?  
If you can tell me your observations we can go from there. Also, what is the ambient temperature when this happens, and does it happen when the temps are moderate.
Roland