After 10mins stopped good boost

[Frank8065]

First of all, thank you for this forum and all the people that keep the info coming,
I have a 2015 t660 isx set @450( or 455),
I have a small issue with the turbo boost, when I take off the 1st time , I go thru all the gears (13spd) and my boost is right at 38 or 39, after the speed gets up there and the boost goes down the next time I get on the throttle the boost wont go past 28-30 psi,even if i downshift, however if i stop for a few minutes , lets say 5-10 mins, while my engine idling I can hear a pssst sound coming from the egr, after that sound i take off amd the boost is up to 38 again only for about 4 or 5 mins ,and the issue keeps repeating everytime I stop for a few mins or go down a long long hill where i am off the acelerator, I already changed the exhaust back pressure , looked at the IMAP sensor and another pressure sensor that sits right by the egr, they where fairly clean, I have warranty on it but im trying to learn this isx stuff, this is my first isx, i have access to insite, there are no codes, really need some ideas here, truck has 150,000 miles on the truck

Thanks a lot, specially to Rawze!!!!!!!!!!!!!!