Boost not holding.

[zurud]

I have problem with boost not holding om red top sr20det. I check for boost leak holding it at 30psi and it hold fine. I have aftermarket ecu that come with my swap. On 1st and 2nd gear I will only manage to get 8 psi and its holding. On 3rd gear upward it will go to 18psi but drop down to 10 psi by 8k rpm. Anybody know why I can't get the boost steady? I shimmed the wastegate, thinking maybe the spring is getting old. I am using greddy profec b boost controller. I did compression check and I get 130psi even on all 4 cylinders.

Thanks