It is the toolpost mechanism that's bent, allowing the toolholders to
unseat. Specifically, two of the three T-shaped plates that pull the
toolholder down onto the ways was visibly bent, preventing full clamping
of the holder to the post. The T-plate in the least-used position is
unbent. No wonder the holders became unseated under cutting force. The
"bolt" does not appear to be bent.

I figured this out 5 minutes after running the lathe with the new
overhead fluorescent lamp operating. Before installing the light
fixture, I really couldn't see that well, and didn't realize how much I
was missing. The toolholder was always unseating, but not enough to be
seen before, but enough to cause all manner of machining problems. The
fixture was $40 from Home Despot.

I have pretty much given up on the Dickson-style toolpost that came with
the lathe. It's just too beaten and worn and damaged to be worth
fixing, although the attempt has been educational. I have ordered the
smallest Aloris BXA set.

Joe Gwinn