one question, is this a gasoline or LPG truck? If LPG,it sounds to me like a possible tar build up issue in the air mixer/injector. Remove the inlet line from the converter to the mixer and see if there is tar/debris built up on the "last chance" screen it will be under the LP inlet nipple,can remove the two small bolts that secure it to the mixer to take it off,just watch for the small clip that holds the screen in place.