My temperature gauge keeps redlining then dropping below normal operating temp. I had my thermostat replaced, still happens, flushed rad and added head gasket repair (liquid) and ran as directed heater on full. Tenp gauge stays low while driving with lots of heat but when idle at light temp gauge goes up and no heat. What would cause this? No bubbling of antifreeze, both fans running.

Did they purge the air? Trapped air in the system acts like that.