Ed1534: I'd say you've worn something out. The end of the speedo cable is square & fits into a square hole in the cluster. If the edges are worn, it could be slipping. My suggestion: replace it.
The inner part should be able to pulled all the way out, but not from the end you're working from. The other end only. Sounds like it has stretched & is about to break. As I said, replace it.
Brumby Boy: your cable needs lubrication. Whatever oil is in the sleeve for the cable to spin in has run down & therefore the cable is catching. Remove the cable assebly from the car, pull the cable out & relube it. Dunno what with, talk to your local mechanic. This is actually a pretty common problem up there due the high humidity & heat in summer.