Diagnosis
This is the electronic regenerative braking system engaging inconsistently, often due to a faulty throttle sensor or connection.
How to Fix It
Power the scooter off. Visually inspect the throttle assembly on the right handlebar. Check for any visible damage or looseness. Gently rotate the throttle through its full range, feeling for a 'dead spot' or gritty resistance. Clean around the throttle with a dry cloth to remove moisture or debris.
If That Didn't Work
The issue may be in the motor controller. Perform a reset. Turn the scooter on. Simultaneously press and hold the power button and the mode button for 10 seconds until the scooter powers off and back on. This reboots the controller.
Heads Up
This surging can be more pronounced when the battery is above 90% charge, as the regen system has limited capacity to put energy back into a full battery. It is a known quirk of the system.