What is happening to my power steering?

I have a 2000 (W plate) Xantia HDi, which I bought when it was four years old. It has given me over 85k miles of mostly trouble free motoring and I'd like to keep it for a while longer. Anyway the power steering is now playing up. What happens is that usually very early on in each new journey the steering will become excessively heavy and stiff. Yesterday it happened whilst I was turning out of my street into heavy traffic. Scary. Within a couple of seconds of this heaviness occuring it goes back to normal. I'm thinking (cos I'm smart like that) that the power steering is on it's way out. Can someone advise me?