I have a 99 f150 4wd 5.4 w/ a weird electrical problem .
accelerating down the road (doesnt matter what spd.) the tach and spedo will start to go wacky eng. will run rough trans will go to neutral , then stall , in order to restart while moving , put in nuetral you have to turn the key all the way back before it will start, but it will restart without problems. this prob is continuous. no matter hot / cold. tried a new reflashed computer, tried new ign sw . by the way there are no code's in the comp. or any other module. problem can be duplicated sitting still by power braking.

 

Sounds like it could be a speed sensor problem in the transmission, especially if it's going into neutral by itself. Or, there's another transmission issue tied in with the magnetic pick-ups on the speedo & tach.