Truck is a 92 w250 auto with 150K on it. The only engine mod is 3.5" FLowmaster exhaust from the downtube back.
While driving at a steady speed, the truck will hesitate, then surge, hesitate, then surge over and over again. When accellerating, it runs as smooth as can be. It starts easy and runs great otherwise.
Changing the fuel filter had no effect on the problem. It is not connected to a certain tank of fuel either, as the truck has been doing this over the last several thousand miles.
What do I check next??????