I checked all four pullies - the cables are securely contained within each of them. There just isn't any visual sign of a problem with the regulator, but it's definitely off-kilter, which seems to be causing it to bind as it goes up. It's true that regulators aren't very expensice, and now that I've been through this troubleshooting experience, it should take me no time to install a new one.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT