Ford mondeo losing/lost power?

My ford mondeo loses power all the time. Cannot accelerate, keeps stalling. Its a 2001 petrol model. Have had the filter/pump , new engine..took to ford garage who hooked it up...no prob showed up, so mech took for a drive and said thought the prob was with filter pump, but have had them changed and no luck...
It is an absolute nightmare to drive..does anyone know what could cause this?? Most of the parts have been replaced and im running out of ideas and money!!