Diesel Bombers - chevy 6.5 problems

Hi,new to this forum,I,m located in England,,and difficult to get answers over here!!I run a 96 ,6.5 chevy 4x4,which ive owned for about 1 year,truck is brilliant,and never lets me down,until now,we had a fuel problem,,and it turned out to be a collapsing suction line from tank to pump,,so ive replaced that,cleaned and tested electric lift pump,all ok,truck ran fine initially,but i then noticed she was pushing a little black smoke on hard acceleration,which has got worse,then better,then worse,so she,s overfuelling badly.i then noticed she seems to almost switch off power,then restores,when running down the road ,this is intermittent,odd times from start,she idles smooth,then if i blip the throttle she idles roughly at around 1500,if i switch her off,then restart she seems fine,she,s still blows balck smoke,though it seems not as bad,ive read that the PMD can be troublesome on these,this one has had a relocate kit fitted allready and the PMD is Stanadyne 34583,ive ordered a new one.With all this the turbo is boosting when it should.(I have a boost gauge in cab).
Around this time ive changed the engine oil,using a 15-40 fleet oil,,is the oil grade critical in these chevys?
I also run a Ford F250 2003(which i,m getting rid of),which runs like a bag of nails if the oils wrong grade,I had tons of problems with this truck with the variable vane turbo,In the end I rebiult the turbo and actautaing mechanisms,got the proper oil in her and she runs strong again,,although having been through the turbo set up,i cannot believe its so complex for a work truck,,to many variables to go wrong,from what ive read these trucks have been notoriously bad,from day one,.
So i,m sticking with the Chevy,we run a plant business,and she,l slog all day in quarrys etc,never gets stuck,,so apart from this glitch i have with her at the moment,am i on the right track??:c:,,any advice gratefully recieved,thanks Tim