Speedometer driven gear issue

[msinc]

Sorry for the delay fellas in getting back to this post.....just yesterday I jacked her up and removed the tail shaft housing so I could install new gears. I installed a black 7 tooth right entry cable drive gear on the output shaft and another new drive gear on the cable. This time it lasted a little longer...I got about a mile and a half out of it before the speedometer quit reading and went to zero.
I kind of knew that the 16 tooth driven gear would read a little high and it did, but at this point I just wanted to make sure it would work. The plan was if it worked and read and didn't fail I would worry about the correct tooth driven gear later. As above, it didn't quite work out that way.
Initially, when I first got the car it read slow but the needle was smooth and consistent and it read something, even if it was off. The second try {this is try #3} when I just changed the driven gear it read dead on and was smooth, but only for about 1/2 of a mile or so. On the second try it damaged the drive gear also, so I ordered both gears new. Try #3 lasted a little longer and read a little high, but it also did something else the entire mile and a half, before it failed...the needle on the speedometer had a steady bounce.
One thing I noticed on try number 2 that I didn't think anything of until after it quit, was that the driven gear seemed to need the cable pushed in a little in order to get the clip on it. Looking back, it was as if the inside cable was a little too long for the jacket. So, this time I took a Dremel and removed about 1/8" off the end of the cable. The gear went on zero problems, the clip went on no problem and the gear and cable turned by hand very smooth. I really thought I had it this time until I saw that needle bouncing.
I am lost...the cable turns very smooth, the speedometer reads until the gear strips and the gears go right together and seem to mesh just fine. I would like to have a working accurate speedometer and this just isn't rocket surgery....but it's enough of a job pulling that tail shaft that I wont do it again until I have a smoking gun. Any suggestions or ideas are greatly appreciated. I have photos of the recent gears, but they really look the same as the first ones. I will post if anyone wants to see. Also, the only thing "different" about this set up from others I have worked on {not a speedometer expert, but this aint my first rodeo either...} is that the toploader was put together without the factory gasket between the main housing and the tail shaft housing. That would change the relationship of the drive to driven gear, but that little bit hardly seems like enough to destroy gears this quickly....1/16" or less???