Back lash means play in your carriage when it moves. like play or movement in a gear train, usually its the X adjustment. I have never had to adjust on any machine I have owned. Are you sure the belts are tight, and are all the pulleys on the motor and other end tight? Use a allen wrench to check.
Did you physically check each mirror for play or just look? Same with the lens. Remove the lens and look at making a note of curved side up or curved side down. Gently clean and replace the same way it came out. Usually curve side up.
My other suggestion is check your wiring connections.
Scan Gap for a 300 dpi image should be around .06 - .08 do some testing to see which is best.