Transmission needs 1hr warm up routine?!

Stumped...haves a 2007 Buick rendezvous
3.5L v6. 220,000+miles. Everything has
been well maintained as was a company
vehicle prior to my purchase.Have all paper
work passed on to me with car stating
anytime service,oil changes,tires/rotations
were done.On to my current issue...check
engine light on and one morning had an
issue after sitting all night.i have to let my
car warm up nearly an 1hour in neutral or
drive with brake on,so that it will drive/shift
smoothly as should.once it is warmed up it
shifts fine and don't notice any issues.if I
turn off and say run into store I come back
out within hour it still drives good.however if
it sits longer than hour iof two I have to start
all over and do the pre warm routine to drive

off.so i had shop look at they told me wiring
harness possibly but dont think that fix
possibly.quote me close to $1000job guess
work.so got second opinion.suggested and
did fluid and filter change.showed me fluid
no but or shavings,however was pretty dark
blackish.but that still didn't bring the tranny
to shift without warm up.it still at 20-25mph
pushes 3000rpms and wont come out of
until warm up hour n couple laps around
blocks.guy ran scan tool and this time came
up torque converter code.so he suggested
do a new torque converter.thoughts on to
save this ride or what it could be?anyone
heard of this???it is the only code that
comes up other than like loose gas cap
code or exhaust leak code.runs and starts
right up.so other than warm up had ran
great for me for the 3years owned under
me.last thing auto repair shop did was clean
my throttle body so would start with ease
said was dirty,when did oil change and
lookover.it took a couple of key turns start.