I’ve been battling with my cooling system for a while now. When I bought the rig (97 Blazer with a 4.3) the coolant was pretty mucky and the heater didn’t work. I spent hours flushing the system and replaced the thermostat and the clogged heater core. I had great heat for about a week and when I was driving yesterday the heater went from hot to ice cold coming out the vents. The temp gauge stays where it’s supposed to be and I’m not losing any coolant and there’s no coolant mixing with the oil. I did notice the temp gauge does rise and fall about five degrees going from a stop to under a load. I’m 99.9% sure I’ve got all air out of the system, and it’s bizarre that the heat works then goes straight to cold after driving for a little while. Any suggestions would be helpful as I’m pulling my hair out on this one.