E320 A/C blows out defrost duct when accelerator depressed
 

Most of the time when accelerator is depressed the 2 middle vents will stop blowing & will start blowing through the defrost duct somtimes even seems to be blowing out somewhere else other than defrost duct being that I dont feel air coming out. When foot is lifted from accelerator the middle vents miracously starts to blow again. The 2 outside vents are not affected. Also, when system is on high it seems to kick into highest speed with a delay.Any advice would be much appreciated.

Air Flow Changes In Response to throttle opening
 

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).