211,000 miles, battery light came on in line at the drive up, increase rpm and it went out, flashed occasionally on and off going home. Tested it with DVM and was able to confirm 12.5 volts with battery light on and 13.5 volts with RPM and battery light off. (Thanks Franklin2 and all others that help teach at FTE). Installed a new NAPA alternator 14.51 volts at idle and 14.41 with high beams and high fan, good to go.
Question is just for fun I had NAPA test the alternator and it tested good, retested three times and it tests good.?

Thanks FTE

 

Your old alternator had a intermittent problem. These types of problems are sometimes hard to re-create. The alternator may need to be hot before it would fail, and it may never fail on Napa's test bench. Same with ignition modules. They are famous for working when they are cold, and flaking out when hot. They can test these too, but it's hard to catch them when they are intermittently failing.

 

Thanks for the reply and further education.