It is heck getting old - I do not remember which it was but when the monovalve went bad in my 300D I seems to remember that I did not have heat while driving down the road but I did have heat while sitting at a redlight (idling) or it may have been the other way around. They are easy to renew - remove the four phillips head screws on top and pull the unit out and swap it. Make sure the engine is cold when you change it out. ALWAYS be careful disconnecting those plastic electrical connectors on these old cars - they get brittle and will break off one of the tabs when you disconnect it.

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Jim