Well,this is what I used to do is work on engines and diesels, but I could sit here and tell you what it could possably be, but that would only be a guess.It could be oil leaking past the turbo seals, could be injecter O Rings leaking, TPS sensor, allthis and others could be the problem. If no CEL light is on, the only thing to do is to put an engine analyzer on it, that way the technician can watch every sensor. Also it will show all the pressures if he has the right diagnostics.
I have seen it so many times before, but I will say it again, process of elimination is expensive!
best to pay for a qualified diagnostics with some one that has the knowledge and equipment to do it with, in the long run you will save money.