Elegoo Nept 3 Plus x Axis Mayhem

Hi,Printed the head just great. Next print of body (no changes to settings) printed first few layers perfect then wildly shifted x axis and stated printing off to the side (off the build plate and wanted to go further than possible). Noticed when I stopped the print the nozzle moved as far to the left as it would go an then grinded because it wanted to move off the printer. Weird thing is this only happens sometimes and I never change any settings. Belts seem ok, I think.Any ideas appreciated. Thanks