Took my S41 out for a run last Friday, within 3 blocks started to surge, buck, backfire etc., within 6 blocks, so bad had to turn it around and bring it home. Extremely loud, thought I had an airleak whatever. Started to take it apart, went to remove the battery, + terminal was loose, explained the wavering neutral light. Charged it up, put it back, took it out today, smooth as butter. Now all my terminals have lockwashers.
Why did it loosen? who knows.