Random movements towards end of job vCarve->GRBL 0.8c mm->UGS (Mac OS X)->Xcarve 1000

GRBL is correct for the XCarve, however Inventables uses their own modified version. The newest version is 0.9j. 0.8 is pretty old so you may be having an issue there. This thread will tell you how to clear the Arduino and flash the Inventables GRBL. There are links in the thread.

As for UGS the current nightly build seems to be the most stable. Older versions seem to do odd things when sending larger files. I believe 2.0 is the current version with 1.09 being the current stable version.

I would suggest starting with UGS and GRBL upgrades and see if that solves your issue. If not you can move on to adjustments and then g-code issues.