Temp gauge rises when AC is used, return to normal when heat is turned on.

What is your question?

Temp gauge goes to about 70% when driving on the highway when using the AC. When I turn on the heat the temp gauge goes down to normal. Seems to happen on warmer days. The radiator is missing about 30% or more of its "fins", not sure if that is the problem or maybe the thermostat? Car has 104000 miles on it.