Most likely an alternator regulator problem, but you need to make sure the engine grounds are clean and tight and battery connections are also the same, if they all check good then you need to have the charging system waveform checked just to confirm that is a regulator problem. Don't put this off, pulsating voltage is very hard on today's electronics.
I was thinkin loose ground or water entry. my older truck did that with water in the lightswitch and towing harness.
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