What year is the Mondeo? I had a 2008 140 estate, it never cut out on me, but many members on the Mondeo forums used to have problems with cutting out, especially when decellerating (approaching junctions, roundabouts etc). The cure was usually a visit to the dealer to have an engine management update flashed to the ECU.

If it's not in warranty any more, remove the engine cover and check all the various connectors on the engine are snug and properly connected. Also check the boost hoses from turbo to intercooler, and intercooler to inlet are properly connected with no oily crud seeping from the joints.