I talked to a mechanic friend of mine and he said he has seen this problem several times.
There should be a single small gauge wire with a plastic connector on the alternator. This connector gets loose or corroded and causes this intermittent issue.
His fix has been to go to ford and get new pigtail end to replace the connector and the issue goes away.
I haven't seen the connector myself but I would think that you should be able to get in there and clean and tighten it.
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1998 EB Expedition 4X4 200,000+, 1999 Windstar 200,000+ (Needs Head Gasket), 2008 Chev Colorado (Company Truck, Really sucks but it's free)
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