[Peter Serio]

I agree, try removing the center spinning section of the cable, clean and lubricate that first.

That's a really long cable, I think it's 118" or 121" inches long.

If that doesn't cure the jumping then by process of elimination, it has to be in the speedometer.

You say it's jumping form 30 MPH to 100, that's a pretty big jump. A dry or worn cable will cause the pointer to waver and bounce, sure. +10, +20 But +70 MPH jump I can't recall ever hearing of or seeing a cable that dry.

Try the cable first and report back. DO NOT USE White lithium grease on the cable!!!!! Try this: