Alternator Problems?

I have 99' Pontiac Bonneville 3.8L 'K'(no supercharger.)
One day, it is running fine for about 40 minutes with no electrical symptoms. I shut it off, after about 20 minutes I get back in my car
and nothing. The gauges, radio, and lights worked and looked normal and bright. I get a jump from a passerby, the car starts fine. I took it to my local "Advance Auto Parts Store" they test the battery on the car and it passed. I started the car and pulled the pos cable off the battery --the car stalled instantly. I took the alternator out of the car and had it bench tested --it passed.
By the way, my serpentine belt is less than a year old with about 20k miles on it. I am driving my wife's car until I figure this out. No need to be stranded on the interstate.
Any ideas? Could it be the battery, starter, or alternator?
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First you can't disconnect the battery in modern cars. The battery filters out all the noise spikes generated by the electrical system. All the noise sends the computers into shock and you will be lucky if you don't damage something. Check for loose connections and corroded connections. You need a voltmeter and diagnose it when it is failing.....

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It was a loose battery terminal. The positive side bolt was stripped, for what ever reason, I got a new bolt, and it runs like a top!!!
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Sounds like a ground problem, run a jumper from block to frame, frame to cowl, radiator support, and any sheetmetal that,s carrying current.
Verify negative battery cable ground is good using VOM, and testing for voltage drop
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Sounds more like a starter problem. Solenoid sticks.

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