There is a flapper on the fresh air side of your air filter, under the air filter box. Were something to be lodging that flapper valve mostly closed, it could limit the amount of air through the filter and on into the engine. But checking the manifold pressure during the experiencing of your issue, would let you know if you are getting a good supply of air to the engine.

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Turbo300Mercede
87 300D W124
83 240D W123
80 300 TD Wagon W123, 4 Speed from 79 240D, SLS Rear Suspension