I'm afraid if you're getting undervolt warnings that's almost certainly the cause of any serious issues like kernel hangs. The lightning bolt not being visible simply means that the undervolt condition is not sustained, but nevertheless any such warnings indicate that your power supply is dropping below 4.7V (usually under load). Before investigating this further, we'd need to eliminate that as a potential cause.
I'm afraid if you're getting undervolt warnings that's almost certainly the cause of any serious issues like kernel hangs. The lightning bolt not being visible simply means that the undervolt condition is not sustained, but nevertheless any such warnings indicate that your power supply is dropping below 4.7V (usually under load). Before investigating this further, we'd need to eliminate that as a potential cause.