K8800 printing problem

At the start of print, I have a sudden shift in my print. The belts are not loose at all, so I don’t understand what the cause is. Here you can see the video:video

Does the sliders move smoothly?
Do you feel sudden resistance?

The sliders moved, but not very smoothly. I added some extra fine oil, and now it runs fine, thanks.

I have encountered another issue. Sometimes, with a fast travel form one point to another, the printing nozzle scrapes over the printed part, sometimes resulting in magnets of the print head detaching from the rods. What can I do to solve this?