Take a voltage measurement at the spindle drive if possible. Even if you have a fully charged new battery, a high resistance connection between the drive motor and battery can give the symptoms you're seeing.

If there's a Cannon plug, sometimes you can insert an Xacto knife blade or similar in the slot of the male pin to spread them just little. Also put some Dow DC4 or other silicone dialetric grease on the contacts to help keep them gas tight and corrosion free to avoid resistance build up and voltage drop.

bumper