truck does not want to stay in overdrive
it could be your tps the throttle positioning sensor. the tps went out in my truck and it had an awkward idle and it kept shifting out of overdrive at about 65 miles an hour. it could just be a gutless truck too. try unplugging it and see if it runs the same. if it does then you need to replace it.
is it a doge truck? i heard of that problem b4 on them, have the valve body on the trans checked and the computer for sure.