Intermittent throttle acceleration sticking?

I have a recently replaced throttle body assembly which came with the idle sensor and throttle position sensor attached. I have new spark plugs. What's happening is when I push hard on the accelerator pedal the engine will speed up like it's supposed to but when I let off the pedal it does not throttle down. My only fix is to put the car in neutral and then the RPMs will drop on their own or sometimes I have to push the gas pedal down real hard 2 or 3 times and then the RPMs will come down. Then I put it back in drive and keep going. The pedal is not sticking, and neither is the new butterfly in the throttle body assembly. What else could it be?