When traveling on the highway, 50mph to 70mph, and removing my foot from the gas pedal, the truck does a small lunge and drops speed, but retains the new coast speed. It doesn't happen every time, but its frequent enough that it needs to be fixed. Any idea on what could cause this?

 

What year, engine, and transmission?

 

2000 F150 (duh) 5.4 with 4r70w