ISX sudden boost psi change

[Carter]

I’m currently driving a 2015 cascadia with an isx15 cpl 4583 450hp 1650 torque. I normally accelerate through the gears (10 speed) with a light foot and hold the boost at 10psi. Occasionally on flat ground I need 12-15 psi boost to accelerate. Then in 9th gear at about 45 mph and about 1300 rpm there is a sudden increase in power and my boost drops to 8 psi and the truck preforms much better. Seems random but happens if I stop and idle for say 10 minutes but not all the time.
Truck is completely stock with 465,000 miles. Not sure of the gear ratio but it turns 1250 rpm @ 60 mph. It’s got me puzzled.