Why is my 2012 Chrysler 200 heating up beyond the 3/4 on the temperature gauge?

What is your question?

The problem only seems to occur when driving more then 5 miles on the freeway, driving through town most of the time the temperature is at a normal lever or just a little high but when I get on the freeway and go a steady speed it rises to the 3/4 possibly even higher. Talked to my mechanic and he was saying that if it dosnt overheat when parked and running they most likely won't be able to diagnose it, and it's not overheating unless I'm driving on the freeway. Never have seen any coolant leaking, reservoir was full so was radiator, so I'm not sure what else to look for. Thought maybe a thermostat but don't want to go buying parts that might not need to be replaced

What makes this problem better or worse?

Worse when driving more then a few miles on freeway. Better when driving through town

How long has your 2012 Chrysler 200 had this problem?

A few months