I had a similar problem when my 97 'E300 D had 10k miles on it. At 2500 RPM it would surge. Todate with 92 k, not a recurrence.

My problem was the two vacumn valves above and next to the exhaust manifold. Apparently the heat from the manifold was ruining the valves. I crafted a heat shield which I attached to the manifold and have not had a problem since.