have u checked your return overflow check valve on the injection pump? if its stuck open it will cause alot of fuel to go back to the tank. worth a try if it helps any.

 

Yes, that is the one, banjo indeed. The isn't another one.

The check valve/pressure regulating valve/ return overflow check valve are all the same thing, just called different things by different people.

 

if the check valve comes back ok, I'd look for a bad fuel connection or maybe a bad seal along the line. my guess is that you're sucking a lil air somewhere.

 

my dads 01 did something simular dunno about the boost cause his truck has no gauges and bone stock but simular some times it would run fine and then it would lose power and have a light blue smoke somtimes and ended up being the injector pump was weak or bad and was just barely letting it run at times dunno if this is any help but after he changed the pump no probs since

 

The Overflow valve will show in a fuel pressure check. In all honesty it sounds like a weak VP or a boost leak that dosent occur till higher boost levels.
Take it to a qualified tech and have them diagnose it. At this point you could spend all year working on it, and we could walk up to it and have it fixed by the end of the day (most of the time). Any qualified shop has the tools and the know how to properly diagnose one of these trucks while most folks dont have either. Not saying anything bad just calling it like it is.

 

My 99 has 346000 miles and derates right around the 2500 mile mark on each oil change, loses just enough oil pressure to derate. Didn't show a DTC for a long time. I went through most of what you did and about fell over when I changed the oil and it ran great again.