I similar issue, turned out to be the watesgate

 

To explain, the wastegate itself is a flapper valve inside the turbo (in our case with stock turbo). When it is opened it allows exhaust gas to bypass the turbine and enter into the exhaust system directly. It is opened and closed by the wastegate actuator. The wastegate actuator has a spring and diaphragm inside of it. The spring puts pressure against the diaphragm ~ 14psi of pressure and this holds the wastegate shut in the turbo. To overcome this pressure boost pressure enters the diaphram and pushes against it. If you had no boost controller then it would take 14psi of boost pressure to overcome the spring and open the wastegate. With a boost controller you can delay any pressure entering the wastegate actuator untill your desired boost threshold is reached. That is why, if you disconnect the wastegate actuator from a boost source, your wastegate will never open and you should overboost.

 

I know this is an old post but I'm having issues and this solution doesn't fix my problem. I have a ix mr and I have the Injen upper and lower ic piping, 3" ic, ebc, hks exhaust camshaft, turbo back exhaust, no cat, arp head bolts, e85, fic 1450, walbro 450, arashi 25g turbo, just replaced my stock exhaust manifold for a buscher stock ported manifold, tuned for 30 psi on stock ecu.

Fixed my boost leaks,(was able to hit 24 psi 1x after that) then just 20 psi from then on. I'm gonna recheck my stock header to see if it's leaking from the head or the turbo mounting area. Am I missing something ? Any one have this issue before and then fixed it?