This was a very well known problem on my E28-not sure if the X works the same or not but the problem is identical. The rubber diaphragms in the heater valve would develop rips. At highway speed the pressure in the cooling system would be high enough to force the coolant through the rips and back into the radiator--essentially bypassing the heater core. As soon as the engine speed was reduced the valves started working normally again. They used to sell just the rubber pieces but then stopped. Had to get a new valve at that point.

Good luck.

Edit to clarify that the valves and sensors always were working-just that the piece inside meant to divert coolant to the heater core failed.

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06 4.4i
01 330cic