Any testing of the generator must be done while the problem is occurring, otherwise it will test good. With the mileage you listed, you can expect to find worn brushes inside the generator. They will cause an intermittent no-charge condition, and it could go on like that for months.
I don't know which brand of generator is used on your car. If you can see it and you see "Nippendenso" on it, I can post some pictures of how to replace the brush assembly if you can find one. Most people just replace the entire generator. I can also share how to test it for worn brushes but you'll need a test light or an inexpensive digital voltmeter, and you'll have to wait with the testing until the problem acts up.
Don't overlook the possibility of snow or water getting on the serpentine belt and forming ice that makes it slip. That would occur at engine start-up, not after you've been driving for a while.
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Monday, January 31st, 2011 AT 8:32 PM