power steering problem

I have a A2 1.4 petrol SE with only 18,000miles from year 2000, in very good condition, but the power steering dash light sometimes flashes warning sign and also on random occassions steering goes heavy then returns to normal. any help in suggesting likely cause please.

Hi A2 people, one day i had i power steering error message on the screen, and it became stiff. Looked for some independent specialist (cheaper then Audi). Found 4 Rings - Dean seemed nice guy, they diagnosed (£90) - Power steering pump fault. He quoted £870 to fix. Looked up the part...