As the weather has warms up and I actually start using the Vent and A/C option on my '86 Mustang I have noticed something weird happening. When the engine is under medium to heavy throttle (average to low vacuum) all the forced air for the interior HVAC system goes up to the window (defrost position). Idling at a stop light or under light throttle, I'll get good airflow from the vent (or floor) but under medium to heavy throttle, everything goes to the top of the dash!?!? Has anybody seen this before? The car runs great, no loping or surging idle - so I'm a bit skeptical as to a vacuum leak. Any ideas or where to look would be great!