It doesn't move until I get up to about 40-50, then it bounces all over the place.  I'm not sure if it's the gauge itself or the speedometer cable.

Is there any way to find out if the cable is causing the problem without replacing it?

Sure. Disconnect the cable. If the speedometer doesn't work at all, it's the cable.






























Sounds like a snapped cable to me. You could always just try lubing it.. but I bet it's snapped.

Does sound like a snapped cable. Unscrew the cable from the speedo and try and pull the inner out of the cover, if it comes out you'll see the break. If it won't come out disconnect it from the drive at the front wheel and pull it out from that end and check the condition of it.

speedo cable is $15 from Ronayers.com
the cable is super easy to remove/replace.
If it is snapped you will know as soon as you remove it.

http://www.ronayers.com/fiche/300_0284/speedometer_-_tachometer/speedometer_-_tachometer.cfm?man=su&groupid=3180&parent=3070