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If what you report is exactly correct, the alternator has a faulty diode or rectifier component, but this is intermittent.
The output must be checked under load and with an oscilloscope or a bar graph multi meter as the second choice, and for a period with mechanical stress to show up anything loose. As mentioned earlier here, heat is also a very relevant factor.
I doubt very much that an auto electrical facility would carry out this sort of test for AC voltage, and if for free with due diligence.
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Trevor, New Zealand.
As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! 
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