2010 Crown Vic PI with accelerating problems.

I recently purchased a 2010 Crown Vic Police Interceptor which I am running as a taxi.
There is a dead spot in the accelerator around 38 - 43 Kph. The problem is especially
evident when starting up a hill or turning onto a road from a stop. Just today I have
eliminated the possibility that the problem is related to a throttle positioning sensor.
Using a friends Crown Vic I swapped the TPS, then the entire throttle body, then the
whole accelerator pedal assembly with no success. I am now beginning to think I may
have a possible transmission problem which I dearly hope is something minor such as
a tranny flush and filter change. There is only 169000 Kms on the vehicle and no code
warning lights except for low tire pressure. Thanks