Tom, most vehicles circulate coolant thru the heater core at all times, and use a blend door to regulate the temperature. If the seals ( usually foam) on the door deteriorate, it can allow excess hot air into the ductwork.
However, I doubt that the blend door is your problem as the seals are the same whether at high speed or low. I suspect something causing the engine to put more heat into the radiator, and the incoming air flow past the condenser is insufficient to take enough heat out of the A/C system.

__________________
1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags