Im pretty sure thats exactly what the problem is, is the wastegate chattering open and closed, not operating normally though (its a distinct oscillating sound like its just flapping open and closed and gets increasingly louder as you stay on the gas). This phenomenon starts around 10-15psi though, it isnt at target boost trying to regulate it. It holds 19psi dead nuts on everywhere else though, doesnt overboost anywhere with the current setup.
And yes I agree that it wasnt the EBCS's "fault," but when it comes down to it the tuner had originally recommended just going with an MBC on the 2.0L, I chose the EBCS because I had heard so many good things about it, but in the end I probably should have just taken his advice (for my setup, Im not saying this is what everyone needs to do). It definitely hits harder with the MBC so Im not complaining. Im not looking to point the blame in this thread or make any suggestion about the EBCS's performance, I just want to sort out this issue. For whatever reason it just wasnt working out on my car when I had it. In the grand scheme of things I am very happy with the car, it just has this weird little issue.
So maybe his advice on tightening the WG actuator would hold it shut "harder" and prevent the WG from opening and closing like that in 5th. I guess Ill have to try it...just wanted some opinions.