1973 Chevy K10 Step Side. I'm having issues with the heat. I know I have a small Coolant leak somewhere, but not sure where at this point. My issue is the heat in the truck works fine when at idle. As I increase speed to around 30-40mph the heat turns to cold...and I mean cold. Like the temperature of the outside temp. I would think if it was a heater core problem I would have the problem all the time. Anyone else had this problem? The system is full of coolant. Thanks!