my car loses its power steering

when im in a low gear the power steering seems to lose its strength, it becomes difficult to steer, also when im driving at 50mph+ my car seems to strain to accelerate, like its over revving, it feels heavy to drive if that makes sense. it 1999, ford fiesta zetec. with a service history, steering fluid level is fine, oil is fine, waters fine, brake fluid level fine aswell, what could this be?

Greetings lauralou, welcome to the Forum !

Usually, when in Low Gear, the revolutions of your engine are greater, and should make the Steering easier. Not sure what to say about that !

Mis-timing of your Distributor is frequently the cause of sluggishness on acceleration.

With the onset of cooler weather, you might try getting the Air Pressure up in you tires, that can cause a heavy feeling of the car.

Have a good Day !
S.W.