Jobs ruined after long run

[BernieNUFC]

Hi

I try to do as much as possible before bothering people by reading/watching video etc but i am out of my depth here now.
Running a 720-Z for 8 months now slowly learning about it and using vectric vcarve pro.
The other week after running a small shield in 3d finish it got to 99% and suddenly did a large L and at a 6mm depth straight through the work.
Yesterday it ruined another piece when after about 1.5 hrs running it suddenly overrun the x axis by 20mm and carried on as tho nowt happened.
Today i started all a fresh and again 99% 40 mins in and it suddenly plunged lower and finished the last run too deep and offset by 5mm and ruined the Pine wood.
Running it at 44mm/sec .25 ball nose in pine.
The only common denominator seems to be 3dfinsh but its all simple work nothing out of the box.
Where could i start to track this fault (but i cant read g code) as i cant waste any more wood as its all for charity and it costs me.
I have only done small sorts of work until these shields and never had any problem, but i am only a hobbyist and do little work with it. I also read here and agree with a previous poster about anything wrong with leadscrews etc and it has to be shipped to Germany and man it costs, a bit of a downer, but cant see why it would be these as no backlash or owt, it just has a moment of its own and goes off on one.
Its on a dedicated PC (win 7) no screensavers or power save options and no internet connection to distract it


Many thanks for any pointers Bernie