Speedo behaviur question

[kejj]

My speedo works fine for a while and then staring to act up.
Basically I've come to the conclusion that it works fine for 1 hour or so appropriately and then starts showing lower speed figures. After a brake it would be fine again for a while and start acting up in shorter time.
If put at rest to cool then it would work for about 1 hour again.

It does not stop working, just show lower speeds and jump up and down between actual speed and lower figure.

Should I suspect the speedo or the worm/plastic part on wheel?
I have this speedo: http://www.scooter-center.com/en/produc ... ENTE_TACHO

[HxPaul]

I've yet to see an after market speedo that works properly for any length of time.

[EddieStone]

Have you oiled the cable?