The only thing that I can think of is a minor flex in the shaft. It is a very thin shaft, about 5/16 inch or so and it is split in the center for the buterfly. The Mustang shaft was significantly larger. A small mis-alignment of roller bearings can cause the shaft to bind; not so with ball bearings. I did not do an engineering investigation but I did spend $350 for a new Mercedes throttle body that did not solve the binding. The price of a little knowledge is sometimes high.
190SL, 230SL 5-speed, 95 E320 Wagon, 01 E320 Wagon, MGB, Boxster 'S', 190SL "Barn Find"