my 98 f150 surges between 30 and 50 mph. change the plugs i put the bosch +4 in and checked the u joints drove it long distance gets 15miles to the gallon . the check engine light does not come on am trying to figure this problem out without pulling the plugs backout for this is along process any help would be great thanks

Possibly something with the fuel lines or the fuel pump itself?

check the OBDII for a transmission code. Buddy of mine had the same issue and it was a transmission

if it has separate coils for each plug it could be a coil.sometime coolant drips on the #4 from the bypass hose on the intake.

The surge may be caused by the torque converter. Ford has had quite a few problems with this. Flush the fluid twice and see if it goes away.

DO NOT USE A POWER FLUSH ON A FORD TRANNY!

sounds like a C.O.P. ( coil over plug ) problem to me. Might want to change your o2 sensors too. Usually around 80K to 100K they are junk any how, they might be the culprit.