2001 Cadillac DeVille. System no charge code.

Good afternoon I have a question I'm pretty good
with cars and working under the hood and I'm an
electrician apprentice as well.. I have a electrical
issue with my Cadillac DeVille it's a 2001 and its
reading service no charge the battery. I have my
alternator tested and my battery and both are
good.. when I start the car it runs and the voltage
reads anywhere from 14 to 13.5. then it will slowly
drop and lose power and I will lose my dash lights
and my headlights and it will read on the
dashboard system has no charge.. then
sometimes when I make a sharp turn all the power
will come back on and all the headlights and
dashboard and I will have full power.. what could
possibly be the issue? Someone please help me
out with advice I really don't want to take it to the
shop I want to figure this out myself and I know it's
not the alternator or the battery.. I'm taking the
negative post off while the car was running and it
kept running so I know it's not the alternator and
AutoZone check my battery and told me the
battery was good.. it's just weird I will slowly lose
power and it will say system not charging and then
when I make a sharp left or right turn I will gain
back all my power