I've not changed the arb itself, although it has polybushes (powerflex though).
It has a very big arb - 29mm from memory - as it's a very late car, so I've been told it was probably just a parts bin special from the end of the Sierra run.

It's a fairly recent thing - definitely wasn't doing this before. I thought the leaking PAS rack was the cause, so was really pissed off when it was just the same after getting a new rack.