Haven't had any speedo problem. Mine was a bit loose at both threaded ends of the cable. Try tightening it up at those points. I found out about the looseness when cleaning the lower end of the cable and found a fair bit of lubricant leaking onto the cable. The other end was also loose. You should be able to get your hand in there under the dash/guages without removing any plastic. Otherwise it could be broken, get on to Zongshen USA - if you're in US. The new cable shouldn't cost much.
