im putting my money on it being the EBC. especially if it is a greddy, I have a mate with the exact same symptoms at the moment. It shouldnt be the wastegate unless u are running stupid amounts of boost through the car. And if it is the actuators sticking the wastegate shut, shouldnt the boost be creeping up not down?
Id say put on a manual boost controller, test to see if it changes how the car drives, and if it is the EBC then look up how to properly tune the EBC. There are threads in here somewhere.