Check the wire that goes from the electronics to the stepper motor. If one of the wires becomes damaged the motor will act like you describe. I bought a second hand v3, and had to repair the wiring as mine was doing the same thing. Damaged wiring will cause it to oscillate at low speed rather than taking small steps.
If your wiring is damaged at the motor connector, then its probably easiest to buy some spare cables and cut the end off both the new cable and the Autotrickler cable and solder the new connector back on. Those little connectors are a pain to work with, and I found just replacing the entire end was easier for my meager soldering skills.
Stepper Motor replacement wiring. Adam also sells cables.....and dang-it if he doesn't describe this exact thing under the replacement cable description.
"If the motor is shaking in place and not turning, the most likely reason is one of the four wires in this cable is broken. Check both ends of the cable and try swapping the cable between the AutoThrow and AutoTrickler to narrow down the source of the issue."
Autotrickler spare motor cable
