240D engine surge after downshifting
 

My '82 240D with reman MB engine has a strange problem I would appreciate any help on explaining before I take it back to the shop (still under warranty). Thanks to this forum, I resolved a transmission flaring problem after my shop wanted to put in a new auto transmission (just needed a modulator adjustment). Now, when I floor the pedal at 40 mph to downshift, its a bit sluggish and at about 45 mph it noticeably surges forward - up to that point I can see exhaust smoke in the mirror which ceases as soon as it revs higher. It does not happen consistently, but frequently. There is no problem with a steady acceleration and shifting through gears. Any ideas? I couldn't find anything similar on a search of previous threads.