Update on this issue: Yesterday Brent from Robust graciously reached out to me and offered the following possibility:

Since you’ve confirmed the inverter is not tripping out, I suspect it’s the switch inside the headstock that determines if your spindle lock is all the way down. Sometimes with heavy cuts, the spindle lock will jiggle inside the headstock and tickle the switch enough to shut off the machine, but not enough to engage the spindle lock of course. When the headstock settles down, the spindle lock sets back down on the switch and the lathe can restart. If this is the problem, the solution can be found in your manual, the very first paragraph:

Failure to Start, try these six steps in order:
1) Push down on the spindle lock: it may have moved upward just enough to open the internal safety switch but not
enough to lock the spindle. If the problem happens frequently, try tightening up the screw that puts tension on the
spindle lock. You’ll find it in the threaded hole directly below the spindle lock knob

This explanation sounds plausible and may well be the issue I've been experiencing. Unfortunately I have flown the coup to southern climate, so it will be awhile before I'm back in my shop and able to test this theory.

Thanks again to all who have offered valuable input, and to Brent for his recent help.