Buying from afar

Speedo was disconnected when I got the car, so its just a simple matter to reconnect it. right? Reconnected it and its jumping all over the place. I previously posted the speedometer was only held in place with 1 cross threaded screw when I got the car, so unknown history behind that. Anyhow I just installed a new cable today, thinking try the cheapest thing first. Guess what, still jumping so its the speedo. Leaning toward replacing it rather that try to rebuild. Under the best of circumstances I'll have a slightly cross threaded housing and a used but rebuild speedo.