My heater isn’t working well. It takes a while to get up to temperature. When it does and starts heating it will only stay warm/hot while the vehicle is moving. When I come to a stop light, it starts to blow cold air and if lingering long enough the temperature gauge starts moving up towards hot. I don’t think it’s the heater core. Could it be just low coolant level or a bad thermostat? Should coolant be a 50/50 of coolant/water? Similarly, was having a problem in Summer with gauge going toward overheating. Any advice is appreciated.