Help needed please on my 1997 1.2 16v ztec Automatic

Hello every one, i only p{Bad Word: Moderated}ed my test about 8 months ago and bought a ford fiesta,
Its a 1997 1.2 petrol 16V ztec automatic 5 door.

I need help and was wondering if any one could give me some answers?

The car starts up fine no problems, when the car is in (Park) and you put your foot down the revs wont go over 4000 and will keep bouncing up and down between 3000 and 4000, this is stopping the car from going over 60MPH on the motor way. I could like with it as I am not a speed freak, but now its failing to hit the 50MPH on the motor way, it struggles to reach that speed so i took it to a mechanic and he plugged it to a computer and told me its a 99% chance its the Air flow sensor, I was wondering if any one ells has had this problem or herd of it?
Any info would be great oh and the guy told me the part costs £150! Like i said im a new driver and ain't got a clue so any info would be good.