Hello Richard,

It has been some time since I have been around trolling motors but it sounds if the variable speed sensor or switch is either corroded or burnt out. Since you had the speeds and they are slowly disappearing it sounds if the switch is failing. If you are somewhat mechanical and know how to test using a volt ohmn meter you can check the switch this way to see if it is the switch. If the switch tests fine then your circuit board in the motor head or foot pedal has gone bad.

Hope this helps some