If its not a grounding problem, then Amp may be overheating. Try installing a fan on the amp or relocate the amp so it can get decent air ventilation and cool down properly
It also could mean that the Amp simply cannot handle the power output.
i.e. My "700w" sony Amp kept on shutting off and dimming the headlights. I replaced it with a lower wattage Audison and it has never shut off or dimmed excessively, and sounds much better.
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