2002 Dodge HO Diesel Question

I have a 2002 Dodge 3/4 truck. High Output engine. At a constant speed, no load on the engine, it surges, up and down. The tack nor speedometer show the surging but it is very “hearable” in the cab. I know a diesel “cycles” when first started/it’s cold, but this isn’t the case. It’s doing it on long trips with the engine warmed. Any ideas or is this normal? Thanks

I have owned two diesel cars and neither “cycled.”

Do you feel and change? I am suspecting it is more a clutch or transmission issue. Does it change road speeds at all?

There is absolutely no change in RPM’s or road speed. I’ve changed the fuel filter and it has a clean air filter. What I meant by cold cycling is (and it’s more noticable at night) is until the vehicle is warmed up, you’ll notice the headlights dim and brighten slightly. It doesn’t last very long. I’ve owned 3 diesels and they all did it. But this is different. You hear this surge,surge,surge sound. Very constant, very exact,perfect rhythm. It does it at any speed, 60,70,65mph. As long as the engine is not under a load, just cruising along. This truck gets terrible fuel mileage. 10-12mpg. It’s a 2002 Dodge 3/4 ton Diesel, 6-speed High Output. 3.55 rearend. 60,000 miles on it.