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Lately, at times, around idle my cars rpms would drop down below 500rpm and the car would stall.

Now I have a voltmeter on my turbo timer that monitors the battery voltage. Most of the time, the car runs fine, and the voltage stays around 13.5 - 14.2 even while idling for longer periods of time.

However, with out any sort of rhyme or reason, sometimes the voltage will drop down below 13 to 12.2 - 12.4 and it will not charge up. When it does drop that low, is when the car starts stalling, and it doesn't seem to start charging again until I get the engine above 3.5k rpm.

Thinking about the things that could cause it, number one the crank pulley I have on is a machined ITR pulley. Which the alternator diameter is smaller, so it is underdriving the alternator. However, if it always was underdriving it to the point of stalling wouldn't it be a constant problem? But it would make sense that once I rev the engine, it spins the alternator pulley fast enough to start charging again.

I am also curious if somehow the belt is slipping at times. The belt seems tight enough, and it definitely doesn't squeal at any time. (A problem I had before with a lose alternator belt). But it would make sense, that at some times the belt starts slipping so it is effectively acting like I had no alternator.

Or I guess it could be just a bad alternator? or going bad, which is why sometimes it works and others not? I honestly don't know a ton about alternators and how they function internally.

Thanks for any ideas/insight in advance.