I have a slightly older MB(see signature) with a similar problem. When I accelerate, the air volume from the center vents drops off. Once the accelerations tapers off, the air flow returns to the center vents. During all of this, I never lose air flow to the side vents. My MB has a fuel economy gauge on the dash. I've noticed that when I accelerate , it pegs and that's when I lose center vent air. When this gauges needle drops back more to the other side, center vent air returns.

It's been said that a gas engine develops more vacuum at idle or slower speeds. As you accelerate, there's less vacuum.

In my case, there are a number of vacuum pods within the dash that control air flow, etc. I suspect that what I have is a weak pod. I also get air flow out of the defroster area when the defroster cycle is switched to off, so I have one or maybe more than one weak vacuum pod.

I wonder if your model is also equipped with these vacuum pods?

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"