MartGSR

My boost control is with a gm boost solenoid with Aem EMS. I don't use boost error correction, but anyway, my waste gate duty cycle map is set at 80% so it's minimum boost. The wastegate is a 38mm tial with 10psi spring.
So what is happening, is when i'm on the road, and I do a pull for example in third gear from 4k to 8500... its boosting 10psi from 4k to 7k, but from 7k and up, it creep at 15psi. And yesterday, i went to the 1/4 track (trying to log and watch the boost creep), and from a little slip clutch (i was with street tire) the boost stayed to 10psi in all gear.
What can cause this ?