robert78.5

A friend phoned from the highway today, he was going to city with his 2004 3500 , ran great on highway, when he pulled into city at about 50 kph, the engine started to surge. He can speed up no problem, but it won,t stay level at around 50kph. I never got any codes from him and he has no gauges. Any ideas. He installed new fuel filter last month.