I'd undo the cables and see if it is them, the twist grip or the throttle body that's sticking or is stiff. Isolate each component and check each individually to find the culprit. Cables can wear the inners and create a groove, so keep the cables routed when testing them, as straightening them out will allow the inner cable to pop out of any groove and seem free.
My money is on the throttle bodies, as they are tucked away and not regularly cleaned and lubed. Cable inner wear a strong second, assuming your twist grip is free to rotate.