1993 Chevy Cavalier Heat

Heat heats up in between 116 degrees to 130 degrees in the car at a idle, and at a idle the heater core hose going in to the core is in between 150 to 160. Coming out its in between 120 and 130. I have changed the thermostat and flushed the heater core with sno bowl. I have also done the head gaskets and sent the head off for inspection / Machining. I have also done multiple coolant flushes to this car. Ok but when you drive the car down the road the heat goes away. The temp reads at less than 80 mostly 70 degrees. Why is my heat going away? What can I do to fix it. I also have no combustion leaks, I have used my block tester on it. Help?