I know it's electronic, I have one of those speed sensors. So it can't be a cable binding problem.
As for the motion I'd say that that it's kind of a gentle lowereing of the needle. It never goes higher than the speed at which I'm going, but goes 10-20 mph lower then comes back up. this normally only happens around highway speeds.
ALSO, if I'm driving with my cruise control on it will sometimes slightly dip andn kick the cruise off, so do you think it could be a vacuum issue?
any input is usefull thanks