2002 explorer v8 4wd eb high idle and shifting problem

This has been going on for 3 years. The truck will be running fine and then it will feel like its slipping in the tranny and not going into the next gear. Like your accelerating but the power is not getting to the tires. At the same time, if you put the truck in park it idles around 2500. Turning it off and on does no good and disconnecting the battery doesn't work. The best way to drive it in this condition seems to manually put it in 3rd. It will stay this way for 1 - 5 days then will go back to normal while your driving and will then be good for 1 -4 months. I took it to a dealership and there are no codes and they have not been able to figure it out. Any info or suggestions would be greatly appreciated. Flt - rick