i have a 2002 gt350 japan model ( i am in nz) done 100,000 km.
in auto at random times will down shift sometimes 1 gear or will keep going down eg in 8 and will drop to 3 , the auto brings it back up sometimes sticking in a lower gear.
if in manual mode i can see it dropping down gears and flick it back up but will keep down shifting eventualy car switches over to auto again even tho in maual mode.
sometimes it will go a week or two without fault and others will do it every day.