Well I was thinking just vac leak in general, which would include hoses. Your gasket is probably fine.

But I have another obscure, better fit scenario (I feel like Dr. House)....

You have a vented BOV and it's sticking. That is why it'll still idle fine sometimes (not stuck), why it's lean (stuck open, sucking around mafs), and why it's fine under boost (being forced shut). Get it to idle shitty, then cap your hand over the bov.

As far as CTS or MAFs, the CTS will just make it run really rich. The MAFs could be reading lean, but you usually get a failure and a 2500rpm limit. I've seen bad O2's force the ECU to self-learn into obivion (lean), but I think that was just a ROM tuning test gone wrong.