Exhaust leak, loosing boost

[v8volvo]

Sounds a little to me like a sticky governor or worn pump plunger, though as mentioned above air ingress could be a factor too, since air will play havoc with everything about how the pump operates including dynamic advance, internal pressure, governor behavior, etc.

I did see something one time that was unusual on an old 1.6L diesel VW injection pump, with a great many miles on it, where over time a low spot had been worn in the main plunger. The result of this was that the control collar wanted to hang in one position, right on that low spot, and the engine revs would stick there (I think this was around 1500 rpm, depending on engine temp) until you disturbed it by dragging the engine back down to idle with the clutch or by making a large control input with the accelerator. So internal IP wear is a theoretical possible cause for a hanging issue or other non-linear response issue like this.

But before considering that, ruling out all the common and easy variables is definitely the first step: air leakage in, fuel pressure leakage out, fuel supply restriction, fuel return restriction, fuel quality. You could also run some diesel purge if you want, which usually doesn't make a difference but sometimes does.

And one other thing to not forget about is the internal pressure relief valve, screwed into the pump at the drive end from the top. If this is out of adjustment or has fallen apart, it makes the engine run weird since it throws the internal pressure curve off which controls dynamic advance, etc. It is a rare issue but the one thing that can help cause it is operating with a plugged fuel filter -- which I think you did experience one time not too long ago if memory serves? Running with the fuel restriction sucks the pieces of that valve apart and then the retainer can fall out and the valve spills its guts into the bore. No big deal to fix it, you just carefully rescue all the pieces from the hole and reassemble the valve (being careful to let no dirt inside!) and reinstall. If the basic checks don't give you any progress I would check this next.