This is a fairly common issue on all the 3.6 ecoboosts. My brother's F-150 had this issue too, out of warranty, it is just a replace it and it is fine sort of thing. I know they did some redesign on it and I haven't seen it come up on the expedition forums but it can display a variety of symptoms. Stalling, dropping to idle while crusing, going into limp mode, etc. Almost always seems to have a code indicating something is up with the ETB. I think the ETB is like 5-600 bucks. Some speculation that it was oil getting into the ETB and ruining it, some said adding a catch can kept them good longer.
