Experiencing racking (I think)

I have a self-built CNC out of 80/20 aluminum extrusion 3030 beams. Approximately 1200 mm x1400 mm table. I have 2 Nema 23s 425 oz-in steppers running the table with DM542T drivers. Smooth stepper motion controller with a pmdx-126 BOB using mach 4. Ballscrews & linear pillowtop rails.

While moving back and forth the gantry feels like it’s running over rocks. This only occurs while cutting and not while jogging. I’ve retightened bolts and plates, but it still occurs. I’ve gone through countless checks to see if any electronics were messed up, but everything seems to be okay. It looks almost like one of the ballscrews has backlash but the other doesn’t. While off, I can move both ballscrews by hand. One of the seals of one of the ballscrews has come loose. Any ideas? Not sure if you can see on this shot. It just sounds like it’s riding into a pothole and out.