1987 535i Cabin Heater Not Working

Hi, can anyone comment on this problem:

BMW 1987 535i

Problem: Cabin heater starts to blow cooler air when car is moving.
a.. Heater starts to work well when vehicle is not moving.
b.. Radiator, thermostat, waterpump, hoses are all brand new. All working properly.
c.. Pressure test shows no leaks in cooling system.
d.. Coolant remains at constant level with virtually no sign of being depleted.
e.. No blockage exists in heater core.
f.. Engine running very strong with no sign of malfunction.
I've been told by a mechanic that the problem is most likely a head gasket leak.
Does this sound reasonable given the above observations?

Thanks for any feedback you may provide.

Grant