2002 stock 7.3liter with 31 gallon standard tank, 145,000 miles. Last few months the engine has sputtered and surged when going down the road at even throttle or a slight increase. Runs fine when accelerating or idling. No check engine light. Usually runs ok when the tank is full and starts sputtering around half full on the tank. Running with diesel treatment/antigel seems to clear the problem up. temps are warm in the 60s. I dropped the tank last weekend and everything inside looks perfect. Fuel filter is clean and no water in the bowl. I'm thinking it must be the inline electric filter along the frame but looking for advice or input from someone that has seen this or knows what it likely is. One dumb question, the unit inside the tank looks like it's just a mixing chamber and not a pump. Wires only went to the sending gauge. Is the pump along the frame the only pump? Thanks for any help.