2010 tundra 5.7 transmission

I've got a 2010 tundra 5.7 4wd around 100,00 miles
was driving on freeway around 70mph when it
downshifted and stayed to what seemed like 3rd
gear, all dash lights came on took first exit, found a
parking lot, shut it off but would not start, ( kinda
like it was still in gear, even though it was in park)
towed it home, after a few days of  sitting it started
up cleared all codes ( to many to list) drove and
shifted normal for about 20 miles then it did same
thing, only it'll start back up after turning off, clear
all codes and repeat....any suggesting before I
start spending alot of money and chasing stuff?
I've since bought a new truck, but would like old to
be useful and not rot in the pasture