Originally Posted by
John
could be an air leak, check all your connections from MAF to throttle body and all vac hoses.
That was my first thought as well. A leak under high vacuum could cause the wastegate to stay open, when you ease up the leak lessens or stops and allows it to close.
With the age of the car and the underhood heat from 2 turbos it might be a good idea to replace your vacuum lines if it has never been done before.