1999 Ford Crown Victoria

Inspect and test all the following listed below:

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
False air leakage.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard.

Sounds like you ahve the classic torque converter shudder. You can try a bottle of transmission shudder guard to see oif it helps. If it does, have the transmission serviced, including the torque converter and cooler drain also. Those hold over 8 quarts of fluid that need to be changed also. Good luck.