2001 f250 v10 6.8L Transmission problem?
 

hi everyone, I would really appreciate any help;
my truck starts fine, and seems to engage all gears when I am stopped going from gear to gear. However, when i put it in D, and start accelerating, I reach a speed of around 25-40mph and all of the sudden, my truck seems to brake all by itself and speed and rmp drop but I never applied brakes and then it refuses to once again accelerate until speeds drop to 10-15mph after what it seem to get unstuck. Best way I can describe it is similar to downshifting a manual trans without the increase in rpm.