Stepper Motor Vibrations

Im using smothieboard for CNC laser control. For some reason the X axis which is engraving line by line “shades of gray” with semiconductor laser is strongly vibrating. This is influencing quality of engraving in attachement). Do you know what could be the cause of this vibrations? Did I miss some setting? thanks a lot for any ideas what to check or try. (default config.txt(just laser mode on))
thanks, Martin