2000 Taurus high engine idle only when in gear
 

I have a 2000 Ford Taurus that has 75000 miles on it.

The engine idle won't drop down when driving. It seems like after it shifts into third or fourth gear the rpm's drop to about 1500 - 2000. When you take your foot of of the gas the rpms won't drop below 1500 making braking difficult.

I have put it into neutral while still moving and the idle speed goes up a little then drops back settling at about 1500.

After stopping the car in neutral it will drop to about 900 but if you leave it in drive it will slowly drop down but it takes a few minutes to do so.

Any help would be appreciated.