50g

I've had my HP-50g for about a year and gotten fairly comfortable with
it, but there are a few things that I can't figure out.

First, flag 123 (Allow/Forbid Switch Mode) does not seem to have any
effect - or there are certain operations that override it.  Most of
the time the calculator works as I expect it to, but every now and
then I'll start getting strange results & notice that it's in RAD
instead of DEG.  There is another setting that sometimes changes 'on
its own', but it doesn't happen as often and I can't think of it at
the moment.  

Integration seems to force a switch from DEG to RAD sometimes
(regardless of the state of flag 123), but not always.  Sometimes the
switch is (or at least seems) completely random I'll work one problem
and everything is as expected, then the next problem weird things
start happening.

I don't remember having this problem when I first got my calculator,
so I'm guessing it's a [side] effect of another flag/mode setting. 
Any ideas how to prevent unwanted flag/mode changes?

Second, what does the 'modulo' setting in the CAS mode adjust screen
do?  I've never (as far as I know) had a reason to change it, so I
leave it at '13', but it's always been a mystery.

Finally, why is HEX the default base?  Will changing to BIN affect
normal day-to-day operations?  I've switched back & forth a couple
times, since I rarely do anything in hex, but figured there was
probably a reason it's set to hex by default.

Thanks in advance,

Michael