Although I am not intimately familiar with this model, in general I would look for a vacuum leak in the parts which control the positions of the flaps in the air ducts; these flaps direct the air to the various outlets and, if there is a vacuum leak in the system, it will generally show up diring acceleration, when the manifold depression (i.e. vacuum) is lower. There should be a procedure for checking out the system for leaks in the Mercedes-Benz shop literature.
During acceleration, see if you can tell if the air starts coming out of different outlets; for example, it might stop at the dash outlets and start coming out the defroster outlets. Probably the blower is running at the same speed, but the airflow is changing where it comes out.
Hope this helps you.