[GTP Dad]

I would start with the thermostat. It may be sticking especially if it is the original. Even if fairly new it could still be sticking. If you still have the problem then try flushing the radiator as it may be getting plugged. You can easily check this if you can touch the radiator. If one side is cold and the other hot then it is bolcked. Sometimes just washing out the fins and cleaning the debris between the ac condensor and the radiator can make a big difference.