You may need a new cable, but step one is to remove the throttle cam cover by the twistgrip, remove your bar end weight, take the twistgrip off and clean out all of the old gunky grease in the throttle cam and gears, inside the twistgrip tube, and on the handlebar. Clean everything well, reassemble with fresh grease and see how it feels. I have yet to buy an old BMW that has ever had any attention paid to this rather unique setup and all of them feel gross and draggy.