<HTML>I've had the same problem on my Honda ss50, my first thought was the spedometer but it was ok. So I thought it was the cable, wasn't it, the problem is inside your front wheel. Insinde the wheel there's a gear with a metal clip on it that's fals into the wheelcasing so when the wheel turns, the gear turns. (the gear drives the cable) The problem was that the clip was worn loose from the gear over the years so I bent it back to shape (like Dave did), but the problem returned after 2 weeks, so the best thing to do is get a new gear and clip.
Some mopeds have a plastic gear, if yours does it could also be that the gear itself is worn.
P.S. Use coppergrease on your axel, DON'T use lithiumgrease, that turns to glue after a while. I found that out the hard way.</HTML>