I think you may have the pot going out in the foot control. The problem there is the thing is part of the board and they don't sell the thing by itself.
I found one that works. Looks like
When you look at yours, read the specs and see if its like mine. A 1k ohm pot. My problem was with an old 74 pound thrust motor.
You have to take the speed control off, a clip and the thing comes off, easy. Then you cut the wires and splice this in. My 15 year old Maxxum did just like yours. Irregular at low speeds, seemed above 50 percent and it worked good. After the replacement all was good.