Restrictors in power steering lines are not that uncommon. My point about the reservoir and hoses was that you might have installed the problem when you changed them. New doesn't mean that they are not the problem and probably number one suspect since the problem came up exactly when you installed them.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)