I took my new-to-me '76 240D on a little 2 hour road trip from Phoenix to Flagstaff on Saturday. Three times on the way up while going 55mph, I would feel a slight loss of engine power, only to look in the rear view mirror to see a huge plume of dark black soot come out the back end of the car and smoke-screen the poor person behind me. Otherwise the trip there and back was uneventful. What could be the cause of that? I had an oil change recently done and it starts/drives fine. Thanks - Mike

133,000 miles