It's the 5.9 magnum. It idles fine and runs good as long as I'm gentle on the gas pedal. If I stomp down on the pedal it lurches and bucks just like the governor is shutting off the fuel at just above 2000 rpm. It does this more when the engine is under load, like going up hills, accelerating, etc. If the weather is below freezing I don't get the jerking, it's just sluggish. Anything above freezing and the jerking starts again. At first I thought bad fuel or water in the fuel, but that wasn't the case. I've added fuel dryer, fuel system cleaner, and burned through 3 tanks of fuel from different places.
I took it to the shop and paid $100 for them to hook it to their diagnostic rig and they told me it was the pump or something in the tank. It sounds suspicious to me, seems like if it were the pump it would be worse in cold weather, not better.
Any theories would be appreciated.