If you have bar-end mirros, that throttle side sometimes rubs the throttle rubber and kkeps it from returning. In that case, just move the whole perch in a 1/4 inch.
Have you actually OILED the cables? Most new cables are lined with teflon, but the old ones can hang up with accumulated dirt. I just use 3-IN-1 oil in that case. Just keep dribbling and running up and down. ( b o r i n g )
I wonder if you just have weak return springs after all these years?
I don't know the intricacies of your bike (would kill for one, though), but I had a BMW with an advance unit that was small weights on springs. As it went faster, the weights flew out, but when I slowed or stopped, dirt or weak springs kept the weights from contracting for a few minutes.