As stated above check the tensioning of the wipers and spring, also check the AS armature alignment and drive pawl travel:

Relay adjustment app note is here: http://www.pbresource.com/relay/gtb-asadj.jpg

If they're out of whack, you can get intermittent operation... occasional jumps of two positions, skidding, etc.

Watch the wipers as the bonus is scoring, see if the wipers jump multiple positions. Sometimes the quick "snap" of the firing solenoid behaves differently than hand actuation.

Yes, it looks like it is stepping correctly when slowly hand actuated, can you observe it when it is operating in the game?

In game, when the solenoid advances the wipers. it is a more rapid action, more like "flicking" the armature with your finger.