2000 Lincoln Town Car shifting problems

When accelerrating, as in merging into traffic from entrance ramp, it shifts up through 3 gears, then if I let off the accelerator and then try to re-accelerate it does not go into any gear. To correct this situation I have to pull over, place car in neutral, rev the engine, let it go back to idle, place the car in drive, depress the accelerator, and then most of the time the car will continue into proper operation. However, sometimes I must do this 2-3 times before it gets back to proper operation. This occurs almost every time I have to enter an expressway. What's up?