i just bought a 97 f150, 4x4. 4.6l with a 4r70w -CA automatic transmission. when i hit 34mph it bucks insanly like i am popping the clutch or something and will not go above 35. it also slips a bit going from first to second gear. i changed the fliud and filter. any idea why this would happen? i am leaning twards transmission being shot, but i dont know if there is a sensor that might be messed up in the tranny that send a signal to the computer telling the speed and that is defective or somthing. any ideas? i am a part time mechanic, so i can test anything and i know my way around a garage

 

Have you scanned for OBD2 codes?