2002 Mercury Sable bucks sometimes at idle

I have the 24V DOHC engine which acts like it wants to shut off when stopped, and at rpm below 2000 and only sometimes. Above 2000 :frown::frown:rpm it runs great. I have been told it is a vacuum leak. Replaced both lower and upper manifold gaskets, all emission hoses and can't find leak. scan tool shows egr system ok, fuel pressure ok no dtc nor freeze frame codes. Check engine light is not on.