It's your speedo for sure.
I opened mine, with great anxiety, but found that it is not that fragile or complicated.
There is a large outer sleeve that acts as an electromagnet. Don't ask me the physics, that's all I know.
The outer sleeve is slightly pressed in and a center ring (driven by the cable) spins to create the field and turn your needle.
My outer sleeve was loose and spinning, catching, spinning, catching, causing the needle to go haywire.
Voila. A simple bit of crazy glue to reseat the outer ring and it's back to normal.