Just bought this less than two weeks ago. Used car. Great condition. Heater worked great. Until today. We turned it on and all it is doing is blowing cold air. Then we turned it down some so it wasn't blowing as hard. It got hot as we were driving but not very hot. Then it just went cold again. The temperature gauge was still perfect so the car wasn't running hot. We just have no heat. Any have these issues and if so what am I looking at to fix this?