05 PT Cruiser Touring Ed Temp Gauge issue

[MDUBOSE]

I'm not sure what could be wrong with my car but when you drive it the temperature gauge creeps up to the red then after goes back down to the middle. The car doesn't drive bad, all the fans turn on,the a/c will still work( only if you're driving on the highway or freeway). Once you stop the a/c gets warm and stays warm until you drive faster.Maybe I need to recharge it? What in the world could be wrong, once connected to diagnostic machine it doesn't throw any codes out. I never know if the car is truly hot or the gauge is busted. Please help!