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My 4.0 Celeb auto does not seem to want to accelerate especially quickly between 30 and 50, even in sport mode. Other performance is also a bit below par, and fuel consumption is down to 18 mpg.

Is this likely to be the box, or something else?

Sounds like it needs a good service first - give it some new plugs, new filters (esp air and fuel) and see how you go.