common fault on the mondeo auto's, they dont have very good gearboxes

your best bet is find a 2nd hand box and swap it in, but they are hard to come by

repairs on those autoboxes arent economically viable.

you could convert to manual (the parts are quite cheap and readily available over here) but its a fairly big job. you would need:

Gearbox and linkages
Engine ECU
Brake Master Cylinder
Centre Consule
Clutch Pedal and linkages
Brake Pedal
Radiator
Instrument Cluster