I would like to know if the 4311 is sitting inside of a cabinet or similar closed in space as this might be causing it to run hotter on average and over heat during those extreme passages.
I wouldn't worry about the warranty as the 4311 is certified by Denon to run 4 ohm loads. I guess that new 4 ohm setting is the reason Denon now recommends the 4311 for 4 ohm loads.
As the others have mentioned leave the 4311 on the 8 ohm setting. I would recommend setting the front 3 channels to a 40 or 60 hz crossover to take full effect of the lower frequency these 3 speakers are capable of playing( I have my M80s and 3808 at 40hz) and see if the Denon still shuts down, if it does I would consider returning it and try the Onkyo equivalent that grunt has.
