Steering Problem

[hungabunga]

My wifes A170 has developed a strange fault, when turning the car the wheel get's stuck and requires a little effort to get it unstuck whilst turning. It almost seems like the steering lock is catching, but strangly if I hold it in the stuck position for a second it will turn fine. The problem does not happen if the car is stationary only when moving. The steering makes all sorts of creaks when turning. I have checked the powersteering fluid level and it is at the maximum.
Any idea what the problem could be ?

Thanks

Sully