Yup. REAL common problem. The brushes are worn and making intermittent contact. The testing has to be done while the problem is occurring. During their test, they caught it while it was working.
If you can find the parts, you might be able to replace the brush assembly without removing the alternator from the engine, at least that could be done on the older stuff that I drive. I've done this many times on cars I put "bugs" in for my students to troubleshoot.
Here's a photo of what the assembly looks like. You don't have to remove the terminal block as I did in the photo, just the three screws.
Saturday, August 1st, 2020 AT 11:11 AM