Help Me Troubleshoot My Speedometer

[CJ428CJ]

I have about 5500 miles on my car. Richmond 5 speed, speedometer right angle adapter coming out of the tranny and a Stewart Warner Speedometer. Speedometer worked fine until a recent road trip. It started bouncing all over the place and eventually fell to zero and wouldn't move. I took the transmission tunnel cover off and removed the speedometer gear and discovered that the teeth on one side of the gear had essentially been "worn away." They weren't chewed up or anything, just kind of worn away.

I checked the speedometer cable and it seemed to turn freely. I put a drill on the speedometer cable and the speedometer seemed to work fine so I just replaced the speedometer gear. When I installed it, I coated it with a coating of engine assembly lube. After putting it all back together it seemed fine. I could detect a very small bounce, about a 1 MPH bounce, but other than that it worked fine - for about 100 miles. After 100 miles or so, the speedometer bounced around for a minute or 2 and then fell to zero. I haven't taken the speedometer gear out yet to see if it is also worn away on one side.

So, any suggestions on what's causing the problem?