I expect you've carboned up your engine with short runs and lacking in hills. On the two MB diesels I've put big miles on each has hit a point where it felt like it was losing power then belched a HUGE cloud of smoke and then ran like a new car.
On my 190D it was combined with a horrible shuddering, I though the car was going to explode. I drive a 55 mile (one way) highway commute so the engine gets good and hot and burns off any carbon. Short runs around town will build up carbon in the engine especially if you allow the car to shift into top gear and low speeds.
A friend has an '80 300SD and once a year he turns it over to me for a weekend. I flog the crap out of it (I live in the little mountains for central MA) for a couple days and give it back, he's always amazed by the power its got when I return it.

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Sadly Benz-less