I have a stock 2002 WRX with 138,000 miles on it, and I have just recently noticed that the boost when I floor it will build to about .1 MPa and then drop to .05 MPa and then build back up to .1, and then drop again to .05. I have tested it in second, third and fourth gears and it does it in all of them. As soon as it builds to .1 it drops and rebuilds. I can also feel it in the acceleration. I do not know how long it has been doing this, as it is my commuter car and I rarely ever even hit boost at all.
I have done a search on the subject and all of the threads are either modified WRXs or similar problems but not the same. Either way, there are no definitive answers in any of those threads. I have also tried to reference a few other sources and have not come up with anything conclusive. Can anyone help me out with this? Thanks.