Welcome Randal and congratulations on your "new" car! What you describe with the odometer is typical--broken drive gear for the odometer (they are nylon, I believe, and become brittle and eventually fail.) There are some threads on the board that describe a DIY repair (I'll see if I can locate one) but most send their unit out to a repair shop.
If you're not familiar with the search function, here's how it works: Look at the top of the Technical Forum page, far right side. You will see a "button" that says "Search This Forum."
Click on it and when the pull down menu appears, click on "advanced". When the new page opens, on the left side will be a box to enter the terms you are looking for.
Anyway, I'll try to locate a thread for you. Welcome again, and post a picture of your car as soon as possible--it's tradition!
Here you go:
http://forums.pelicanparts.com/porsche-911-technical-forum/196987-odometer-gear-replacement-electronic-speedometers-long.html