Let's see. I'm looking into my crystal ball!!!
I'll bet you anything it's the vent control valve. I just went through this exact experience about a month ago with my B14 (symptoms and fault codes included). For about $100 at the $tealership for the new part and 10 minutes of your time the problem will disappear.
BTW turboprick gas is spelled with one "S" (gas) not two (gass).