I also have an 83' 300D and I've recently been having heating temperture issues.
I'll be driving with the heat on (not fully) ACC cycles fine, Temp is around the 85-90 mark then all of a sudden it will rise almost into the red, then come back down. After this event, I checked my fluid level in the expansion tank it had dropped about 2 inches from the recommended level line, exiting through the overflow hose. I have changed the therm, monovalve guts, Flushed system. I have no idea. Question: should the bottom radiator hose be cool to the touch?