'Lumpy' steering advice

[simonh]

Looking for advice please or pointed in the direction of a relevant thread. The steering on my phase one DT has gone lumpy, like it's being steered with a 50p in the mechanism rather than something round and there's no self centering. There is plenty of fluid (although I have no idea how old it is) and the belt and tensioner have been replaced so no slipping. Sometimes I think it gets smoother the further the car is driven but may just be getting used to it.

What area should I be looking at next? Does anyone know if it's more likely to be the rack or the pump?

Thanks in advance!