I would say that there is a hang up inside you IP. Mine did this as well, but it turned out that all of my linkage ball sockets were dry and sticky. I sprayed white lithium grease into all the ball sockets and it works like a charm now. Nice and smooth action. Returns right back to idle right away. But you say yours does this even with the linkages disconnected. If it were me I would try letting it idle while I got a little rough with the IP linkage arm. Now I dont mean rip it off, but Pull up on it and back down over and over with some force and see if it does any better. I am not sure how these IP's are governed, but it may be your governor sticking or something? Just some ideas.