SERIOUSLY weird 'slipping' issue - need help fast

Hey, Matt here (DIY Perks on YouTube). Got a project coming out that uses the x-carve extensively, and I’m having some seriously weird issues with the x-carve that I’ve not been able to work out.

It’s easier to explain by showing, so I recorded a quick video of the problem:

The idea was to have the project available as a kit, but due to the x-carve acting up I’m this close to giving up, so any help would be very much appreciated.

Not had any problems like this until now - all of the pulleys are secure with threadlock (confirmed to not be slipping), and when the issue happens there is no usual ‘judder’ that happens when the stepper motors are put under too much pressure. A different laptop was tried as well with the same results, and I’ve grounded the sheathing on the stepper cables. Also updated the drivers and tried different tensions on the pulley straps. It happens randomly on different parts of the cut, and usually between z axis changes (looks like stair stepping from the side of the ones I did with more passes). Seems to happen on both the x and y axis.