Yes, as Mr. Laufman suggested, this may be a case of the protective circuits for temperature and/or current being set too low. I recall that about four years ago there was quite a bit of internet discussion about Pioneer Elite receivers(generally quite capable with 4ohm loads)shutting down all over the place. It didn't appear to be fully explained, but the situation apparently was corrected by servicing the protective circuits on the units.

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Enjoy the music, not the equipment.