Sounds like Boost Creep to me, careful in what gear you get into boost with dont want to floor it in 4th or 5th as this could cause boost creep.
I think cobb has a low wastegate duty cycle map and a high wastegate duty cycle map. I think you should try the high wastegate duty cycle map, see if your in line with the target boost(I may be wrong) I am sure someone else will chime in.