Still horse related , but we used to call it "hunts or hunting"
, I think on the old lorries it was due to the mechanical governor kicking in to keep the engine idling, so the revs would go up and down as it corrected the engine idle speed.
If mainly when cold, just started up and for the first few minutes it may be due to heater plug operation or not as is the case, as trying to improve combustion on a cold engine, so could look in that direction. However if still doing it at normal engine temp then not related.
Any black or white smoke on start up from cold?
Have you checked fuel filter for any contaminants etc. It may be a good idea to change the filter anyway just for good maintenance.
Throttle issues usually throw up error codes.
What sort of mileage has the vehicle done?