Just had this problem. I had the whole cluster out to get to my clock also. Mine would jump when just barely moving and get crazy at high speeds. Never knew what speed I was going. I thought it might have been the speedo itself until I turned it by hand. It was smooth and would not jump. While I had it a part, I hit all contact points inside the speedo with a tiny drop of clock oil or it might be called jeweler's oil[?]. Sometimes you can get a kink or a rust spot on cable itself but it looked fine. So I lubed it up with speedo cable lube. I beleve it is graphite and oil mixture[?]. It seemed to bind a little at a certain spot when I reinstalled it. Before going through all the work of reinstalling the whole cluster. The speedo was already off the cluster, so I attached the cable to the speedo itself and laid it in there and took a drive. Still jumped. I have seen rust inside the cable housing, and kinks or dents in the housing. Guess I will need the whole new part. Thought I would try one last thing with nothing to loose. I took the cable out and used 400 sandpaper and gave it a good sanding top to bottom. Cleaned it really well and then used regular bearing grease. It runs very smooth and quiet now.