Is this an alternator problem?
 

Sometimes when I slow down to an idle the voltmeter on the dash drops into the red zone, and after a few moments the engine dies. But I can always start it right up.

I took the alternator to Autozone and it tested fine. But this problem is intermittant. The alternator has 187k miles on it. Battery is brand new and it did this with the old battery.

Sometimes if I rev the engine before it dies at idle the idle will go up to 3000rpm and stay.

I am inclinded to just buy a new alternator but since it tested OK.......

Although most don't drop that far, it is common for voltage to drop at idle. You can follow this write up and get more voltage with only a small investment, less then $10.

https://www.gmforum.com/t278806/

The drop in voltage is severe, sometimes to the point where the engine will die. This symptom started abruptly too. Seems like -10 degrees F could have triggered it....

I am thinking the regulator or alternator is the culprit. It tested OK on the bench, but the problem is intermittent. I despise intermittant problems. I wish the lousy part would just fail so I can figure it out.

How long do alternators last these days? Mine's at 187k, that seems like a stretch but I would be disappointed to drop $130 to find out it's not the problem.

Check the condition of your belt. Make sure it's not cracked, worn, or loose. You may just need a new belt.

Ah, the belt. Always good to check.

It is 2 yrs old and is a Goodyear. It looks fine and I am getting no noise. I will look a bit closer at it while it is running. Good point.

Make sure it's not loose or glazed. Check the tensioner as well.