1988 2.5 Auto
 

My 88 Grand Am runs great with its little 2.5L motor..the only problem I'm having is when i drive the car for about 20min at high way speeds, when i stop the transmission stays in high gear..it won't downshift to first gear...i have to pull over to the side of the road and wait till the trans cools down...if not i have to put it in "N" and start the motor and rev it up and shift it into 1st gear.....
any idea whats going on with my trans?