Speedo needle jumping

Hello Folks. 1981 Pontiac Parisienne. Aftermarket ( I think ) cruise control. Speedo guage needle jumps + fluctuates. Cleaned + lubed cable from cruise control box to speedo -- no change.
Next will do same for cable to cruise control box from tranny. If no change, contemplating taking apart cruise control box + lubing + cleaning inner gears connecting both cables.
Q: Is this possible .. can CC box be disassembled to clean + lube inner connecting gears? There are 4 nuts on side cover where cables connect to box.
The speedo itself checks out OK .
Please reply to NG.
Thanks, -JS
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