Well... a few times over a few months is not terribly reproducible... you can try to get help from Apple, but I'm not sure they'll be terribly useful, because you're not going to be able to reproduce it at will.
I don't think this is a common issue, because we haven't seen it here before, but that said some iMacs have issues with the midplane. Maybe they'll have some ideas when you call -- it's worth a try.
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--Dave Nanian
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