It should not matter whether the sources are connected to the AVR or through the TV via eARC. The owner's manual does make mention of the feature where the volume is lowered when the circuitry gets too hot. I would start with the simplest check and have a look at all speaker wire connections on the AVR and speakers. Make sure that there are no frayed wires protruding anywhere that may cause a short circuit. If the wiring is good, what brand and model speakers are you using? They may be a model that has a lower impedance than what the AVR can handle at high volume. If this is simply a heat issue due to lack of ventilation, you could add an external fan on top of the AVR for added cooling, like one of the AC Infinity AIRCOM units.