Still happens with the ballnuts disconnected, I had removed the ballnuts last week and cleaned them out, then tested that principle as I was originally thinking along the same lines.. The problem can happen at any position on the bed (2mt x 1.4mt) although it seems to happen much more on moving the x axis +, rather than -..
Mechanically, the machine is sound and I have torn apart and rebuilt it twice in the last week, everything is freely moving..

If I get a chance, I will make a little video of it happening later today..

Steve..