My 00 crown vic p71 is having tranny prblem, i need help asap
Wow, this is a broad question and I side with your mechanic on this one. I think a sensor is shot. There are a couple of sensors involved with your car's performance. They, in turn, interact with other sensors so that everything operates in unison. In others words, if this sensor, you can call it the kick-down sensor, if you want to, isn't reading the proper positional information from your throttle -- starting about 1998 Ford was using a great many electronic circuits and sensors for performance, rather than just mechanical systems -- then it isn't telling the ECU (electronic control unit or the vehicles "brain") the proper information. In turn, the ECU can't order other systems to fire or work in the proper sequence.
Given all of this, it is little wonder that your Crown Vic isn't performing well.
I would suggest having this range of sensors looked at, the Mass Air Sensor, the Idle Air Bypass Sensor, the Throttle Positioner Sensors and the like. Granted, some of these sensors (Mass Air and Idle Bypass) are more for idle and low-speed performance, but you just can't tell when one or more of them may fail and interact with other sensors in the change. I think the most likely candidate of the bunch is the Throttle Positioner. If the ECU isn't reading throttle position correctly then performance will suffer.
One other possibility -- and this is a thought -- is to have the EVP sensor checked as well, though this one is more involved with emissions than other things. Still, it could affect your Crown Vic's performance.