There are a few reasons why this could be happening. If it shuts off right away(5-10 seconds) you are most likely dealing with a bad component, probably a capacitor or transistor.
In the case of shutting off after 20-30 minutes, something is overheating or a capacitor/transistor is getting "leaky" once it heats up. It is very hard to be specific as to which transistor or capacitor, but the first place I'd look would be the power amp section and the power supply. One thing you could do is open it up and see whats getting hot, but remember the risk of shock is great. Be careful what you touch. Ideally you are using an infrared camera to see whats getting hot. Any other type of trouble shooting beyond that would require a multi-meter
One other thought, does this unit have a fan? If the fan is running slow or not at all? This could cause overheating.
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