Bucking (misfire) while driving

Hi everyone, beening reading a lot on here and couldn't really see anyone with the same problem as I have, so here goes. I have a 99 windstar SE with about 170km on it with a 3.8L, I been having a problem with chugging,misfire,bucking whatever u want to call it while I've been driving, but only when I'm at a continally speed, if I accerate the bucking or misfire goes away, idle is ok in the mornings or at a stop light, replaced the plugs about a year ago, also I have not gotten a engine light at all so I could read the codes any help would be great thanks in advance this board is GREAT

I'm also dealing with an occasional misfire at startup when warm, but I got a CEL light. Look for clogged EGR ports, perhaps bad plug wires, could also be the coil pack- but I have no idea how often those go bad. Have you changed the fuel/ air filters lately? Try running a few doses of injector cleaner (Techron or Seafoam) and see if that helps.