When warm and after a period of slow driving, the tranny in my '97 3.2 Auto Trooper seems to get confused and will not change up into high gears unless 'booted' and then wants to change down far too early. I stop the truck, open the hood and shout loudly at the engine. I then restart and all is well until I repeat the same sequence, which may not be for a day or two. Any ideas? (other than continuing to berate the truck).