miss fire or trany problem ?

1997 f-150 4X4 Between 30-40mph on a hill or flat surface if I give it just a little gas it feels like the truck is running over speed bumps.Can't tell if it is the tranny slipping or the engine missing. It only lasts a couple of seconds. Took it to the Ford dealer and was diagnosed as water around the plug causing an arc. Drove off and had the same problem. Back to the Dealer...this time plugs,wires,carbon blowout and fuel injectors cleaned..$900. Problem returned two days later. Back to the Dealer who this time decided to change the fuel pump. No problem for about 3 weeks but now it's begining again. Tranny fluid clean and less than a year since changed. Any ideas ?