Have you tried pulling off the air intak pipe at the manifold end? It sounds like air starvation, could be that the pipe is collapsing. Is this a purely mechanical pump like the Sierra or is it electronic?

I've been a believer in Millers Diesel Power Plus/Eco Max since I had a Scorpio diesel that would get progressively smokier until I dosed it, which would clear it out for a while - must have been the injectors sooting up, and it clearly improved it.

That was an old-style diesel - modern high pressure injectors are different but I have continued to use it, even though people tell me it's snake oil.

A friend of mine, no longer with us unfortunately, was a serious fuels expert who worked as a consultant on petrol and diesel standards. He believed there were clear benefits, in terms of reducing unwanted deposits, from the GTL super-diesel fuels and said the Millers went a long way towards the same benefit at much lower cost, so I have used it at every fill since then.