Here is more info on the aforementioned brake booster line - Jump to the TEST in this one.
http://forums.pelicanparts.com/porsche-911-technical-forum/435952-vacuum-leak-you-probably-didnt-ever-think.html
The air valve / vac limiter fitting may need attention and is a cheap fix from Autozone for a PCV baffle for a couple of bucks – the Baffle AZ#5333.
http://forums.pelicanparts.com/porsche-911-technical-forum/671417-oil-breather-line-id-86-carrera-3-2-a.html
The pipeline (part name reference from the PET) behind the TB can be a problem – check for cracks - don’t buy the expensive part – get 2 different diam vac rated lines, fit one length inside the larger and clamp it down – see my picture where I wrote “fixed”.
Get a small qty of vac lines from your FLAPs and replace the lines.
Check the intake rubber that TToversteer mentioned earlier.
Connect all of the lines to the charcoal canister, TB, Air Box, cruise, etc. See vac diagram.
You mentioned that your idle is fine so your idle switch must be too. I would recommend you still check it since the switch could be stuck in the idle position (signal to DME), while your engine isn’t idling.
The ? in the diagram is a cloth braid line vented to ambient air in the vicinity of the firewall.
HTH