95 Ram surging intermittenly

I have a 1995 Ram 1500, 5.9 L, This truck has a surge in it every once in awhile. Mostly what happens just after I start it and begin to go down the road it begins to surge like it is running out of gas, usually it only lasts for a hundred yards or so, then it staightens out and run normally. Sometimes it is everytime I start it up and then sometimes it goes away for a few days or so, sometimes when the engine is hot and sometimes when it is cold. I do get a code 21 sometimes also, which has to do with the oxygen sensor not fluctuating. I have a new oxygen sensor to put in it this weekend. I recently have changed the plugs, wires, dist cap and rotor and coil with no change in the way it acts. It really has me stumped, my mechanic friend put a fuel pressure tester on it for a few days but it didnt act up at all when that was attached, thanks for any help you can give me

No matter where you go there you are!!

it actually acts like it needs a new fuel filter, or like it has dirt or something in the tank blocking something, really confused!!