88 Accord heater blows hot only when moving

88 Honda Accord DX, 2.0 L carbeureted. My heater will only blow hot air when the car is moving. When stopped the temperature of the air blowing from the vents decreases significantly. This happens at any blower speed. Anybody encounter this same problem, or can suggest a solution?

Interesting. This morning (first really cold morning since I got the car), I noticed that the temp fluctuates fairly significantly while driving a steady 70. Maybe if someone knows the solution to YOUR problem it might help me with MINE!

I'm a pretty good basic trouble, shooter, but for the life of me I can't imagine why the temp (his or mine) should vary once the motor is warmed up.

I thought about a vacuum problem, since I think some of the heater mechanisms are vacuum-driven. But the temperature control is direct via a cable.