[roree]

[quote=makippert]My 1991 Aerostar (4.0 w/automatic overdrive) has developed a bad hesitation when shifting from 2nd to 3rd gear (right at 60MPH) but only under full, pedal-to-the-floor, acceleration. It's not just a sputter or a lag, but it feels more like somebody flips off the ignition (or maybe like some sort of govenor).

I have to back off the accelerator slightly for it to recover quickly and complete the shift, otherwise it will hesitate and sputter for several seconds before finally shifting and going back to normal engine speeds.

I did find that if I back off on the pedal just slightly when it's about to shift, then I don't experience the hesitation.
Mark,check the obvious. The wires, plugs and vacuum lines if possible. Throw some techron or Lucas gas treatment in the tank. It may be something simple like a bad wire. If the plugs or injectors are a little dirty the gas treatment should help.
As an experiment I continued to "floor it" and found that when it goes from 3rd to overdrive at around 85MPH, I will sometimes get the same lag. However I don't have the problem going