Its been quite a while since I have driven my truck in a stock tune but I am pretty sure mine did the same thing before I tuned it. I had thought about it a lot back then and now thinking about it some again, and trying to remember exactly what had been going on.
When I first had my truck it would build 28psi, then after a little while it started building less and less and got to where it could barely get to 20 and sometimes jump to 25 or so in spikes but never maintain it. I had my EGR
valve plug up shortly after and I cleaned it but it didn't change the peak boost. I started to think the vanes were sticking but after adding the tuner I can hit 24psi all day at WOT
and easily build 30psi when rolling into it while the converter is locked in OD
My fathers 06 with 75k and bone stock its entire life does the same exact thing, only builds 18psi. Drives perfectly fine and still pretty quick compared to a stock 7.3L so he has never had any reason to actually look at anything since it drives fine and no CEL
Wish I could give you an exact answer but I don't think its a turbo issue or anything in the intercooler system. Only thing I found when doing research on it was some people speculating to an "EPA made" but that seems to be just a myth. I kinda wish the EPA mode thing wasn't a myth cause then it might be a fix, mostly speculating but I think its in the FICM
and cuts fueling. I know my truck won't peg a pyro even on the race file like many others and doesn't seem to smoke as much either, plus was has been down a good 60hp or so compared to trucks running similar mods.
Sorry for the ramblings but hopefully we can get your truck fixed and mine at the same time.