It is my opinion, the speedometer cable needs to be lubed or replaced. If you lube it be careful not to over do it or the grease may spit out into the speedometer head. You don't want to be looking at a greasy speedometer.
I should have pulled the cable while I had the seats out for new covers.
At my advanced age, I can't get under the dash anymore without haveing a rough time getting up. It's getting to be a real PITA (no pun intended) because my instrument bulbs keep popping out and have to be reinserted.
There's a cable inside the sheath. Disconnect both ends and pull it from the car so it's easier to work with. You'll want to use grease; pull the cable out, grease it an put it back in. Clean up the excess at the ends.
I HAVE on occasion been able to get the cable to pull out from the bottom, leaving the cluster intact and the housing connected to cluster... No WD-40 is not the preferred lube.. a lighter weight grease is... not sloppily greasy tho, just a 'skim coat' along the whole cable... Also it would be BETTER to remove the whole thing, to properly clean the sheath of dirt/dried grease... a good spray with a cleaner in the sheath while 'scrubbing' it with the cable in and out and rotating then let dry and the grease the cable part as I mentioned and re-assy..... If you dont get lucky pulling the cable out from bottom... As i said would be better to remove clean and lube.....
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.
Pull it off the trans remove the clip that holds the gear and the cable should slide out. Cover it in regular old motor oil slide it back in spin it untill you feel it engage in the speedo and reinstall gear and back into the trans. Don't use WD 40 or similar use real oil.