Hmm, that sure looks like one or more of the vertical bearing sets might be sticking or is slipping while printing. How many hours do you have on the unit? Have you kept it properly lubricated? Is there any visible wear on the vertical bars? Does the unit make any popping/grinding noise when printing?
I assume that a auto calibration and movement tests all pass without issue, correct? what print surface are you printing to? Magnetic Pad, metal plate with glue or other print surface?
Just some thoughts off the top of my head. Hopefully others will pipe in as well.