What year? The Renix ones have a bell crank attached below, and when the pivot there wears it can hang up. Uncouple the TB from all the linkages, and check it then. If it still sticks, I'd start taking the TB apart piece by piece. You could start with the TPS, see if that has some kind of glitch in it, then remove the throttle plate, etc. Eventually you'll have to find it.