I had a similar problem where an antifreeze coolant leak was dripping on the belt causing the power steering pulley to slip. It took me forever to find! The steering was fine sometimes, "jumpy" when hot. Just a thought.

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John
2001 ML 320
1988 560 SL
2009 Toyota Tundra TRD
1992 400 E (sold '07) (bought back, '13)
2008 ML 350