Sounds like boost creep.
So as I said earlier, there a whole lot of conjecture about what causes it.
I had it, thought it was the wastegate flap hitting the bell mouth dump, but wasn't it.
Did a lot of searching and found a few people with it, but no real answers, just suggestions like bad boost controller etc.
Then one day, A guy on here had it and he got a PFC and had it tuned, and his boost creep dissapeared. It was figured that the rich and retard that you get from an untuned stock ECU with more boost and some basic mods causes excess pressure in the mainfold due to Exhaust gas still burning as it exits the cylinder. Rich and retard does that, kinda like antilag tuning does the same thing. Not everyone seems to get boost creep either.
SO anyway, other people are getting boost creep for other reason as as well, but this was more to do with bigger turbo and bigger highmount mainfolds, different story.
I had mine tuned using a SAFC equivalent and it fixed the problem somewhat.
That is all...