Mate call GCG - Get a heavy duty actuator
The actuator you have now simply will not be upto the task, and the EBC's sometimes just can't hold it.
We all know that, seen it plenty of times now across the forum where people upgrade the actuator and the problem goes away almost immediately.
There is no reason that I've seen yet to suggest a GCG hi-flow cant hold 18psi beyond 4000rpm.
Yes yours is the smaller version compared to the OP6 (and its only about a 25rwkw difference in most cases), but it should be holding a long longer and as i said earlier...
GCG are silly for not forcing people to BUY HDuty acutuators are part of the hi-flowing as plenty of people have had this issue and with hiflows in general.
The stock actuators dont like holding 18-20psi, and then add to the fact they are 10-15 years old.
No EBC is going to account for a failing actuator, and look @ 99% of the thread here about boost control/hiflows, I've yet to see anyone get all funky with actuator setups over and above a normal heavy duty one