X Axis shifting/jumping/skipping while in finishing path

Ok so…

I’ve been milling on the ol’ x-carve machine for a while now.
Had miscellaneous problems along the way and have always problem solved back to fix the issue. The forums on here have been great!!

I’m now in need of some help to which I cant seem to overcome.
I’ve been milling 3d objects in big sheets of high density foam. The cut does all the roughing fine and gets through most of the finishing (x-axis milling only) but seems to get 3/4 the way through it then ‘jumps’ across ruining the whole 4hr job.

I have checked the belts, tensions, wheels, settings, pentos (most recent change) but can’t seem to get any answers.
I have run smaller test files etc and it makes it through.
Have also run different finishing toolpaths (waterline, pencil etc) with no luck

Always seems to just skip about 2-5mm to the right on the X

Thoughts anyone?

See pictures below.

This one is the finishing done right ^

This one is the same file where the cutter has jumped^