my guess would be an air leak, whether that be boost or vac i'm not sure.
have you tried resetting the ecu? the old disconnect battery, pump brake pedal a few times and let sit for half an hour?
i'd also be looking at the mating surfaces of the plenum. i know that it's on the car now, but you can get a sheet of glass and sit the mounting surface on it to see if it is sealing everywhere.
still on the air leak side of things, grab a short length of hose and hold one end to your ear. move the other end around the engine bay and it will amplify any leaks you may not have noticed