[brcidd]

Yes, you have a vacuum leak under ood in the black plastic line that is used to pull vacuum on your mode door actuators. Under accel the engine offers very little vacuum and the doors move to their default spring loaded position which is to put air on the windshield (defrost mode) for safety reasons. When you coast, there is a much higher vacuum available in the intake manifold which now pulls the doors to there requested position (vent -panel or heat floor). So if you trace the black plastic line from your intake manifold to the vacuum reservoir, you will probably find cracked, broken or melted areas that need replaced. The check valve that hold vacuum against your mode doors may also be amiss- so check it for proper operation. I usually remove them and suck on them-- should seal one way but not the other.

This is very common issue on older vacuum operated mode doors- that use actuators. Heat from engine deteriates the plastic lines to the point they become brittle and break.