1997 ford e-350 cargo van stuters internitenly on the highway


hi my van stuters or hesitates (kind of feels like it loses power) when im on the highway. it does this at any speed wether its accelarating or descelarating while pressing the gas pedal or just simply letting it coast. the problem seems to be intermitent some days the van is fine and other days it scares the hell out of me. i replaced the u-joints, checked the resistance on all ignition coils, replaced all spark plugs, swaped rear brake drums, changed diferential fluid, had all four tires balanced when i replaced them with new ones. i even put the rear of the van on jackstands to see if i could duplicate the symptom but couldnt.

any idea if it could be the transmission.

i would really appreciate anyones help.