What is the problem with my 1987 300SDL (198k miles)? -
While driving around town I noticed black smoke from the tailpipe during acceleration and
corresponding reduction in acceleration performance with the black smoke. Since I had
not done any highway driving in for awhile I took it up on the interstate to "blow out the
carbon". The black smoke did not stop but was greatly reduced so I figured all was
better. Two days later I drove from Orlando to St. Pete and about an hour or so into the
trip the 'problem' started again - black smoke with sluggish accelerating. I drove on to my
exit and when I stopped at a red light the car stalled. It would not restart at that time. The
MB Roadside Assistance arrived about 45 minutes later and when I tried to start the car,
of course, it started. I drove, without a 'problem' to the MB Dealership where the
diagnosis was dirty fuel and air filters. So the fuel and air filters were replaced.
On the way back to Orlando the 'problem' happened again. The car ran for about an hour
then black smoke with sluggish acceleration and when the traffic became stop and go the
car stalled. It started again after about a 30 minute wait.
This time I took the car to my local mechanic. His diagnosis was bad fuel. So the fuel
tank was drained and the filter at the bottom of the tank was replaced.
All was well until I drove the car for a continuous hour and, yes, the 'problem' happened
again.

Any and all suggestions as to what is the cause of my 'problem' are greatly appreciated. Thank you.