Carefully loosen the throttle housing at the right grip, split the housing, and WD40 the cable-ends inside the housing. Get some WD40 down the cable too. Be sure there's no debris or damaged plastic on the throttle grip where the cables connect. Re-assemble the housing and leave the screws a little loose. As you do the final tighten of the screws see if the stiffness returns (indicating the housing is crimping thegrip). Even if this fixes it, the next time you have the tank off, WD40 that end also, and check spring installation is correct.