I had problems with my RS once, turned out to be the little washer type drive in the front wheel slipping, which made all sorts of weird things happen, I just bent it out a little and it worked fine and still does after ten years plus. Not sure that's your problem though, I think yours might be in the speedo head itself. I remember reading once about someone who had a spider nesting in theirs which caused a strange problem, have you tried taking it off and giving it a gentle tap or two. I assume you have checked the drive and the cable to make sure all is working freely. If you know anyone with a bike with a cable tacho see it you could connect the speedo head to that, if the needle moves right when the engine revs then you know the problem's not with the head. Actually a bit of old cable in a drill chuck might work just as well, if you're careful. Just a matter of eliminating possibilities I think, as with many problems. Good luck!