Its def not the alternator. Bit odd that at revs the volts drop usually other way round. But still that wat it should be sitting at. Also u have no probs starting ur car in the morn so means its got nothing to do with electrics. Sounds to me it could be fuel pump or something that sorta region. Recently had similar problem and that and afm were the prob. U dont have afm so yeah. I recommend taking it to some1 to have a look. With ur electrics shutting down this could be diff from my prob so yeah. a mech should solve the issue.

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I had the loudest stereo in a wrx in WA. Until i hit a wall.