Gear Box Problem Or May Be Not! Please Help!

Recently I bought Mondeo 2004, 2.5 petrol engine, automatic gear box. The problem occuring is this, when accelerated slowly then can reach up to 240 km/h. But, when push the gas pedal suddenly, the engine revvs, but the speed does not change, as if you are revving on neutral. Sometimes it speeds up but again very slowly and engines revvs up to 5 to 6 thousand before it shifts, but acceleration is very slow, as if it is towing something huge. I have had computer diagnostics - they found that ABS sensor is out of order, so the mechanic said that is the fault - it is not accelerating as it should. Please tell us about any similar problems, if any occured in the past. thanks a lot. Should I change the sensor or is it gear box fault or anything else?