Truck falters and hesitates

My '03 Ram 2500 manual diesel is hesitating and faltering when I drive it.
Sometimes, as I'm accelerating and going through the gears, it will falter like
it isn't getting fuel. Other times, when I'm cruising down the interstate, it will
hesitate for a second and then surge forward, hesitate and surge. The truck
sat for several weeks over the winter (temp never below single digits). The
problem started when I began driving it again consistently in the spring. I
have changed the fuel filter twice (very dirty first time and somewhat dirty
less than a month later) and changed the (very dirty) air filter.  Could this be
debris in my fuel tank? Or in the lift pump which was installed in 2010? I can't
seem to get anyone with a clue to look at it.
Thanks, Patti