Vauxhall astra club 1.6 8v 2002

hi,
just bought this car and it came with a noise, if you move the steering wheel in left or right, short or long turns,there is a noise that sounds like the electric power steering motor,and the lights dim on the dash,i took the car back where i got it from as its still under warranty,they put a brand new steering rack and power steering pump on,saying this was the fault, sorry to say this did not sort it out it still does it,anybody no what might be wrong, would it help if i had the car put on an engine management check, would this find the fault as i think it is something electric due to the lights going dim on the dash. i have also put a new battery on just in case, it made no difference.