driveability problem

My first post.
I have a 1997 pontiac transport with a 3.4 engine. At 150,000 engine failed. It ran fine but something happened to oil pressure. When new engine was put in I had the shop put in a new O2 sensor and a crank sensor because they were hard to get to. The replacement engine used oil so after 8,000 miles they warrantied it. Since getting the van back, it has a intermitent problem. It is most noticible when running a steady speed (usually 55 to 65) then letting off the gas as if to slow down a little, when you go to ease back on the gas it acts about like an older car does when the choke partially closes, (no power, feels like it is missing but no service engine light on) most of the time if you hold it there it will clear up. Or you can snap your foot off the gas and that helps more than anything. Once it starts doing this, it will do it under steady throttle also. Mechanic checked it out and said all he could find was a mass air flow sensor that was a little out of spec. and it was running a little rich. I have changed both O2 sensors,throttle pos. sensor, map sensor, fuel pump, plugs, wires, control module,and egr with a known good egr. nothing has cured problem. all appears to have helped somewhat but not cured. Also had injectors removed and cleaned. And tried a mas airflow sensor but the box had been handled before so i don't know if it was a good one. PLEASE HELP! P.S. I didn't mind to much changing all these parts because I work in a parts store and get a pretty good break on my parts, and i plan on keeping this van IF I CAN FIX IT.

SInce you have taken care of all sensors and ECM, and all this happened on a sudden just following engine change, I'd suspect something like a broken connector , mixed connectors , mixed or disconnected vacuum hoses, a "not in the book" condition. 2 recent cases on this forum mentioned finding such after searching for long about driveability issues.