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Thanks for posting
I would suggest having a competent technician check the charging system/alternator. It sounds like th alternator is not charging or at least doing so intermittently causing the battery tpo drain and then you loose electrical power including power steering which on this model is electrically operated.
Did your local mechanic actually test the charging system?, "doesn't think" seems a little unsure. It either checks out good or it doesn't. So other than that there is nothing I would suggest without having a definite answer about the condition of the alternator with some test results stating voltages and current readings at various load conditions.
Thanks for the update, I'm afraid I can't be of any more help since this seems to be intermittent as you say, they have you vehicle and if they can't find it then I sure will not be able to help without even having access to it, good luck.
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