If you slow the video down to 0.25 speed you can see the left side(closest to the camera) stops first then winds the gantry up like a spring until it is pulled along by the right side.
Does it happen at other positions of the Y axis? Backlash is a possibility as previous mentioned, check belt tensions are equal and the pulleys on the motors aren't loose. Maybe worth checking the electrical connections on the motors, make sure the drives are fully powered down to prevent arcing. If you take the belts off you could move the gantry to check just the v rails/rollers. Permenant marker the rollers to see if they slip while jogging at full rapid speed.y
Did swift-cut ever help with freezing at the m12 command or did they just give up and stop making the machine?
Does it happen at other positions of the Y axis? Backlash is a possibility as previous mentioned, check belt tensions are equal and the pulleys on the motors aren't loose. Maybe worth checking the electrical connections on the motors, make sure the drives are fully powered down to prevent arcing. If you take the belts off you could move the gantry to check just the v rails/rollers. Permenant marker the rollers to see if they slip while jogging at full rapid speed.y
Did swift-cut ever help with freezing at the m12 command or did they just give up and stop making the machine?