Engine is misfiring?

Truck starts fine, idles ok, when I put it in gear the engine starts to miss and run rough, when I press on the gas it smooth out some.. When driving it will run ok at a steady speed then when I start up a hill or to pass and give it some gas it starts to miss until I get up some speed then it smooths out, I replaced the plugs and wires, that didn't help. Any ideal on what is going on. I have no problems out of it when starting or at idle, but when accelerating, only occasionally will it sputter and act like it is misfiring. The check engine light is not on and when not sputtering runs like a champ. I have replaced plugs, wires, coil pack, fuel filter, and added injector cleaner. Fuel pump seems to be fine and all vacuum lines seem to be intact. It has a new air filter as well, but I'm not sure where to go from here. I feel safe driving it anywhere, because on the highway at driving speed it does fine, and only sputters for a second when accelerating. I am on a tight budget and would like to fix the problem before it becomes more serious. Any help would be greatly appreciated. Thanks