I have a bbk throttle body and when I step in the gas pedal and release the throttle body is still open and I'm still maintains the sane speed or even more. But after 3-5 second of releasing the pedal the throttle starts to close slowly. What can be the problem here? Car is a 87 trans am

 

Remove all the throttle/cruise/tv cables from the throttle body, is there any binding? Do the cables feel free? Maybe the cruise control servo is going bad.

 

I believe it only sticks when I am driving. It doesn't stuck when I'm in neutral free revving

 

Sounds like you may need to adjust the blades open a little bit and then adjust the tps.
BBK throttle bodies are known for issues such as this.

 

Would that cause the car to keep accelerating when I let off the pedal for about 5 seconds?

 

Yes it could, due to pressure differential on either side of the blade. Mine would do like you describe. It would also make the accelerator pedal somewhat stiff until you depressed enough for the blades to crack open. I adjusted the stop screw and then slotted my tps to get a good voltage. It's best to set with a scanner so you can see what the IAC steps are at.