Immediate (unplanned) shutoff of Denon AVR 2807

Hi -- newbie. My AVR-2807 (which I love) has developed this annoying habit of shutting itself off for no apparent reason. The only thing that it appears to be nearly synchronous with is a short sharp sound coming through the speakers (I thought it was the subwoofer, but it did it today on a golf ball being struck, and I don't think that was the subwoofer). The center channel speaker (which is relatively large) is on the a/v shelf directly above the receiver -- should I try to put some isolation/damping under the speaker? Is the receiver just flaky? (It's still under warranty from the place I purchased it on clearance last summer....just barely). But what a pain to disconnect everything and take it back........