OK, are you ready?
You need to pull the speedo out for starters.
On the rear of the speedo is a pot metal quick connector that your cable fast connects to.
Now this small root beer barrel sized thing is compression fitted to the rear of the speedo.
When it comes dislodged it can and will move a millimeter or 3 and cause the spinning metal strip to rub the copper flywheel. If this copper wheel is hit just a bit it would jump your speedo. Also understand that this copper bell/flywheel has a 1 to 2 mm clearance between the metal strip that spins around it. If anything from a Beatle or anything hindering this from spinning free will make it jump.
Now what to do? Hmmmmmmmmm…
CRC or WD40 to clean it out and if the pot metal is loose super glue the outer joint or some other super epoxy this is something I have yet to fix but if it lines up it can be loose.
Don’t get spray on the meter face, pain to remove.
And like others have said, it could be something else.