Mine was doing the exact same thing, however, it had been an issue also for the previous owner because he'd had it checked several times. I have the receipts for a new pump and various filters and hoses but it remained. However, during the summer I rebuilt the front end, new bushes tie rod etc. anyway long story short, I phoned my local garage to book an alignment and was chatting to a friend there and mentioned the problem. He thought it could be that dirt had fount its way into the pressure relief valves in the rack. So suggested dropping it off and he'd flush it out. Anyway, he did and all went back together and problem solved. It been fine ever since.