Here's a company that has been rebuilding speedometers for a lot of years. They switched most of their business to computers and small electro-mechanical stuff some time ago, but they used to be called "Ohio Valley Speedometer Supply". Give them a call and see if they still fix speedos.
http://www.micro-tech-auto.com It sounds like the odometer (the mileage part) is locking up and twisting the cable off, or the bushing where the speedo head runs has gotten tight. Oiling it might help for a while, but iot's going to need rebuilding or replacing eventually.
Jerry
