I had a similar problem with my 1996 Renault Megane 1.6RXE (engine cutting out for no apparent reason).
It was solved by a new set of spark plugs and cleaning the connection terminals on the fuel injector ports with a piece of fine sandpaper.
Gove it a try, but I expect the breakdown people tried that already (or at least I hope so !)
Geoff White
October 2005