Why would a 1997 ford fiesta lose power at 3500 revs in all gears?

Assuming not diesel. Could be your catalytic converter becoming blocked

I presume that the car is ok when ticking over? It is most likely to be the fuel pump relay switch, basically the car is not getting enough petrol for you to accelerate but is just enough to tick over. Do you also have a problem with the lights coming on and the fuel gauge not registering?