86 535i. Cold day today. Heat would only come on when car idled in traffic. At highway speed heat would almost cut off, then return at stop light and low speed. Coolant level OK. Temp gauge barely registered at highway speed(rather than normal mid-range), but would increase when car idled. Have checked archives...disagreement over whether it's the thermostat or the heater valve. This past summer did replace thermostat, and lower temp. switches for aux. fan. What do you think? Thermostat broken or heater valve needs rebuild? Or....? Thank you.

Same symptoms one answer, removed thermostat (none there) and replaced. Note; manuals says adjust temp control in small increments, not full swings