3406E derate power after 5 minutes in top gear

I bought this 99 KW T2000 with 3406E 1LW 475 HP with 13 speed 3 months ago and still trying to figure out why after about 5 minutes of driving in top gear at 1500 rpms at wide open throttle boost goes from 25psi to 22psi.
Truck is in good shape with only 570k miles. No check engine codes. New filters and everything. I have stock boost gauge, also Scan Gauge KR edition and also aftermarket Boost gauge. All 3 gauges shows same boost so no problem there. Doesnt better what temp is outside it will still derage. Also if i made quick stop, shut engine and start it, get up to speed and same thinh 5 minutes later boost will drop. Also if im at cruising speed and kick it in neutral and let it idle for 10 seconds and than back in top gear, something will reset that darm minutes thing and it will do again. I tested wastegate with pinching hose thinking spring is weak but turbo will spin faster 3 psi extra so 28psi than 5 minutes later will drop to 25 psi so its not wastegate. I got new cam, boist and oil pressur sensor installed, same tbing
Engine will make more than 22psi at higher rpms 25psi at 1750 but im using 22psi at 1500 rpms just as reference.
Truck pulls great and i know 3psi isnt much but it just driving me crazy not knowing why its doing it

 

Well truck build in 99 maybe is old but back in day when i was building mu 94 Mustang 5.0, ECM had close mode to runs richer for 150 seconds after start and after 150 seconds will go in open mode. And that was 1994 Mustang, so why not 1999 Cat ?

This guy looks like to had same problem 3406E power problem

And if you look at post #17 someone was telling him about glitch in ECU and that he needs reflash ECU

 

DSR trucking that hauls race cars bought this truck new in 99 and they had it for 19 years. Only had 570k miles when i bough it. They made like 30k miles a year. Only info i have is that this truck was only once at dealer back in 07 for some kinda 475hp ECM update