How long do you plan on keeping the bike ?
The speedo housing is accessable, from the rear, and I believe you do have to break the housing, kin of like a seal. If I recall, and if it is like most out there from the 80's and 90's, the spring that controls the movement can be lubricated. I would guess that there is dust in the movement parts that is affecting the uhh, movement. Good luck, and maybe take pics of what you do, in case a question comes up down the line if your seeling your Connie.
Ride safe all,
Greg
