My transmission in my 97 4.2L 2x4 with an estimated 240,000 miles will slip between 1st and 2nd gear to the point where it feels like the whole truck will pop a wheelie, and all of a sudden the next day drive like a new truck. What will cause the truck to slip one day and the be fine? It has been doing this for a while now, how can I fix this? On of my buddies suggested changing the trans screen, but I'm not sure?