You got a big, ole' vacuum leak somewhere in the system of doors which directs the airflow to the various outlets. The default (no vacuum) mode is usually out the defroster ducts. When you open the throttle on a gasoline engine the manifold vacuum drops and there is no longer enough to overcome the leak; therefore the doors move to their default positions.
Look for a vacuum hose loose or leaking, or a rotten rubber diaphragm in one of the duct door positioners.
There is more-detailed instruction on this somewhere, but I don't have any for your model. On my W126 this wasn't too hard to find and repair (did not cost much, either).