Heavy acceleration, engine sputters, then is very difficult to restart

Under light to moderate acceleration, car runs fine, under moderate to heavy acceleration, engine sputters, then is very difficult to restart. The car was stolen from pep boys (was having transmission rebuilt) and was torn up pretty good. Now, it sputters and dies when I give it too much gas. Two shops later (and one was the dealership) I have no answers. The mechanic tells me that he can not get communication with the car's computer, tho it runs fine for hours at idle/light acceleration, no check engine light (yes, lite works).

Replaced spark plug wires, air filter, fuel filter, pcv valve and full 3-part soltec cleaning. Scrubbed throttle intake, but still no difference. Research tells me that the computer is behind the passengers side kick panel, but mechanic was connecting behind the battery on the drivers side? Prior to the theft, the car ran so beautifully that I felt I could drive it for decades to come.

It is the mitsubishi 3.0 V6 and what I would like is a starting point.