1999 s320 steering problem

[jellycream1]

Hi folks,
My s320 steering is fine at idle,whilst stationary,turning from lock to lock with very little effort at all.
When moving though,the steering tightens up,to the point where it won't self centre,and I need to correct the steering when coming out of a bend/corner.
Come to a stop,and the steering is fine again.
Checked all obvious,such as steering fluid level/pump drivebelt tightness...
Any ideas would be most welcome please.
Thanks in advance.