A jumping speedometer is the result of three possibilities.
1.A kink in the cable. The kink may be in the inner cable, although the design of these cables tends to prevent this. Easy to check though, just pull out the inner cable being careful to keep it clean and see if there is a dogleg bend in it, if that is the case replace.
2.Cable needs lubrication, if there is no kink just lube the cable with some moly-graphite dry power as you slip it back in. Sometimes people lube it with transmission fluid, if someone did that to yours, lube again with transmission fluid since using graphite now would just gum it up.
3.If nothing else works there is too sharp a bend in the outer cable routing, sometimes this happens up where the the cable attaches to the speedometer head. In this case try to straighten and reroute the cable with not-so-sharp bends.
One more thing, I usually work from the dash side of the cable. It is much easier to stay clean up top.
Hope this helps.
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