I have an 84 300D 350K miles, well maintained. The heat works fine at idle but cools down when driving. Immediately becomes warm again if you stop. Any ideas?

have you checked your coolant level? Try that first. if that seems fine look at the mono valve insert and scan for any loose or broke vacum lines.

Agreed, check monovalve diaphraghm for damage. Four screws and it pulls right out.