Hunting Diesel

I am hoping for some feedback on a problem with my Maverick 4.2 Diesel.(non turbo)

At around 50 to 60 Kph in 4th gear it surges along or hunts as another descriptive.

Over 60 Kph and it drives along good.

I just had a new seal on the injector pump replaced as it was leaking and I thought this might have solved the problem.... but it didn't.

I use a fuel additive/conditioner for the low sulphur diesel on market at my usual servo and the injector fix it man says this is good as it will save seals etc in the pump. Have tried using another brand fuel for a few fills....same same

Timing was a thought and the diesel mechanic man says this checks out ok as well and couldn't give a reason for the surging / hunting.....mmm no luck here.

This leaves it to hopefully someone on this site who may please have an answer or had similar on any diesel that may lead to a possible remedy.

Thanks
Vinnie