Power Steering Goes Heavy

[Sundayjumper]

Slightly odd one this, the steering on my car occasionally goes very heavy like there's suddenly no assistance. It's not repeatable, but when it happens, it's usually at low speed when I'm accelerating and winding on some steering lock, e.g. pulling out of a tight junction. When it's happened I've always managed to haul the steering round and keep the car going in the right direction but I'm slightly worried that if it happens while my wife is driving she'll end up just ploughing straight across the road

The belt doesn't look too bad and there's no squeaking or squealing noises so I don't think that's the problem. I suppose that's a nice easy thing to change though just in case. Fluid level is fine and FWIW the car had a new steering rack about nine months ago (a while before I bought it).

Anyone else had a similar problem ?