You should maybe check for a voltage drop at/to the starter. Some things to check would be BST connector, splice to starter (I think is at the under hood connector) the connection at the starter itself and the ground connections. I am talking gas so I don't know if the connections are the same.

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