I have a 98 12 valve, #10 fule plate, engine break, 4" exhaust, front of the turbo has been upgraded.
My truck started jumping,jurking, surging ect. seems like it is only doing it when I am towing and at lower speeds under 30 mph, almost like there's a broken throttle spring or something. If I push the clutch in and open the throttle it goes away or if I just romp on the throttle it will stop and run fine for a little bit.
WHAT COULD IT BE????????????

 

fuel delivery issue ... check fuel pressure from lift pump to Injection Pump ... let us know what it is

 

found that one of the throttle return springs was broken
Also TSB 18-29-97 Rev. A talks about the Vehicle Bucking or Jerking while under light acceleration or driving at a steady speed