Speedo Questions

[hot!rod]

Surely somebody has seen this. I can be going along just fine (even though my speedo is 10 mph off) and all of a sudden my speedo will fluctuate up and down from 50 to 90 quickly. When this happens, I can hear a very faint sound. It sounds kinda like a buzz, but it might be the cable vibrating or something. Anyway, then it stops and settles back to being 10 mph off. I figure the reason it's off is due to gear and tire sizes. This leads to my next question. Does anyone know where to get new gears for the speedo? Preferrably the driven gear.

Thanks in advance.