Sounds like something in the Onboard Refueling Vapor Recovery (ORVR) system isn't working properly. You can verify your car has ORVR by looking for option code M664 on the sticker under the frunk lid.
I worked with the system a bit solving my coke bottle noise problem and the occasional hard to fuel problem. There are a lot of threads out there about it. Read this system description and the following thread.
http://986forum.com/forums/boxster-general-discussions/30050-orvr-valve-location-2000-boxster.html
Checking the fresh air valve (11) in the wheel well can be done by listening for the valve solenoid to click it open. Remove the gas gap, stick you finger in the filler neck and push the flappy thing all the way up. This will trigger the magnetic reed switch (9) and actuate the valve. You should be able to hear it.
You can do the same thing for the ORVR valve (6) on top of the tank, but I had to disconnect the fresh air valve (11) in order to hear it click.
I would also check the hose that connects to the housing that the fresh air valve (11) is in. If this is plugged you won't get the air flow you need.
Of course check your E36 fuse.
Threaded Mode
