Could be a bad AC compressor if it does it when the AC or defrost is on. It is also possible that there is a shorted wire on the electrical side that draws enough current that the voltage drops. The AC compressor could be tested easier, just unplug the connector that turns the clutch on. If the issue stops it's a bad compressor.
Without knowing what the actual model and engine is that is about all I can tell you. Please give that information and we can try a few other tests.
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Wednesday, May 31st, 2017 AT 4:19 AM