ford mondeo automatic petrol 54 plate?

gearbox only engage after a few minutes and then the power is lost again.
computer code says torque convertor at fault.
can this be fixed or is it cheaper to buy second hand gearbox?
how much is a new gearbox?
these gearboxes only last about 40 to 50,000 miles. will a better gearbox fit the existing 2004 mondeo engine?
appreciate views to fix asap and as cheap as possible.