170 thermostat, proper coolant mixture, underdrive pulley and no air in the system. Sitting in traffic with the outside temp at 100 the heat goes to 205. I turn off the AC and turn on the heater and the temp starts down almost immediately. Temp goes to 195ish and turning off heater and on the AC, shortly it goes back up to the 205-210 range. Just turning off the AC does not help. Same temp problem as above happens at 80 mph and I can drop the speed to 65 or repeat the heater scenario as above to lower temp. All this in 100 degree weather. Yes, we do have 80 mph speed zones in Texas. Seems to this feeble mind that all I am doing is adding additional cooling coils via the heater. Thoughts????