>    33  (gdb) condition 1 lval == Qnil
 >    34  No symbol "builtin_lisp_symbol" in current context.

I see.  The condition would have been

   condition 1 EQ (lval, Qnil)

but lval is not a C variable.  So it won't work without modifying the
source.  I have no good idea yet how to do that.  Setting these
breakpoints unconditionally is out of the question - we want to catch
the transition from Qfullboth to Qnil here.

Maybe you can try the attached diff (it's against the release version)
and set breakpoints at the four lines I marked with a

// break-here

comment.  Start gdb via run -Q, do F11 and restart CWTM.  Here you have
to switch to the debugging Emacs (either you have it on another
workspace or you find an equivalent of Alt-TAB used by most window
managers) type "bt" and post the backtrace here.  Maybe it will let us
find out why that parameter gets reset.

martin