OK. It did it twice more this morning, and i am certain that the couplings are not at fault. I marked them and they haven't moved.
I homed the machine this morning before i started.

The first time the Y was 250mm off where it should of been when it started cutting along the x direction, and the second time I have no idea what it was doing, but basically it appeared to slow right down and then start cutting in the wrong direction. I was profile cutting 18mm lightply.
In all instances the feedrate has dropped from what i set (1500) to very slow when the problems have occurred, although Mach3 did not show this on the screen.

I have tried going back to the default Mach Screen layout as i was previously using 2010 screenset, and it still did it.

I am trying now a mid air cut, to see if i can reproduce it, and to film what actually happens without any danger.