Machine not following toolpath

Over the past few weeks I have created toolpaths using Vectric Aspite and saved them in format as Stepcraft UCCNC arcs (mm) .txt which is listed in the dropdown list on Aspire. I have then loaded these toolpaths into UCCNC and set up my Stepcraft 2 420 machine as per instructions, the same way as I have done in the past without issue.

The last few days the machine starts of running the toolpath fine, but at some point in the process, sometimes at the beginning or sometimes half way through the process the machine seems to stop and the spindle bores right through the material and would hit the bed, if the estop is not activated and spindle switched off.

Its driving mad and is causing me to ruin some expensive materials.

I am mostly running v carve toolpaths, which all run fine in the past

What is more annoying is its not consistent as other times the same toolpath can run fine.

Help !!