I also have a 1985 300SD with same problem, replaced mono valve insert, thermostat, pushbuttons, temp control unit and aspirator motor. still same problem.
Last week when the aspirator motor seized and popped the fuse I had plenty of heat. Whenever I buy one of these old german cars I make it a point to clean all the fuse contacts and replace the fuses. having a number of these cars in the past I'm pretty well versed but this one has me for a loop. Last night I was driving stop and go, had plenty of heat but once I hit 65mph on the highway again no heat. Pulled off at a rest area and unplugged the mono valve and had so much heat I had to cycle the off and EC buttons to keep from getting cooked.
Has anybody had an experience with a mono valve magnet getting weak, I'm almost thinking that the plunger is getting sucked down at higher speeds and shutting off water flow.