Ever since I've changed my intercooler pipes, I've had the same kind of boost problems. I have my MBC set about 22.5 psi. About one a month (i drive it daily), it will drop and never go past 18 psi. It will just stick there. I either have to take my MBC apart or just turn it all the way up, then all the way down, then bacl to where 22.5 is. Then it works fine again for about a month. It usually happens if I drive for more than an half hour, but not always. Lately, its been happening every 2 weeks. I don't think it is a boost leak because it stays solid when it wants to stay at 22.5. I've tried 3 different MBC - a hallman, forge and the inline buschur.

 

hmm...I was thinking if you were using the forge, I've read of people having the ball inside the mbc sticking. Simply removing it, putting a very light coating of oil on it and reinstalling usually fixed it. But is sounds like you've ruled out the chance of it being a bad mbc..

Are you running an upgraded wga? ie: forge...is that 18psi your wg pressure?

 

stock wastegate

 

Sounds like the wastegate actuator arm might be loose or combination of loose/sticky.

Try tightening the arm a little and let us know if it helps.