I had the same problem on my 2001 4/4 black top zetec. The only fix that worked for me was to install a restrictor plate behind the idle speed control valve. The first I bought from an Austrian Morgan dealer. Now I\ve started to manufacture them myself from beer cans.
In the end I chiseled out the internals of the idle speed control valve, before installing a new beer can restrictor plate.
I made the holes small enough to achieve an idle of 900.
When the engine is cold in cold weather it runs rough, but when warm, very nice idle.
Now I can engine brake downhill, and accelerate as hard as I like without any hanging rev.
