This scenario is known as "surging" (which, if you use this word on a search, you will find threads on this).
It is most prevalent on the W124-series cars, but other models have experienced the same.
There have been numerous solutions to the problem, but none absolute.
The most talked about is a vacuum leak. One of your hoses is split or has separated from a connection causing the surge to occur. This is the simplest (and cheapest) fix.
Another member has mentioned an intake manifold leak that was fixed stopped his surging problem.
Others mentioned the AFM or air flow meter needs replacement. I tried to fix mine this way, but it made no difference.
Still another member had the reverse gear sensor switch for the transmission replaced, and it solved his problem.
When I joined this forum three years ago, my first thread was about the surging problem...and I STILL haven't solved it. Hope you have better luck!
