Well, I thought (hoped) it was fixed... the shop said sticky thermostat and replaced it, and it drove for miles without a problem. Today at lunch, it happened again. I stopped the car and bled the radiator, then it was okay.
Returning from lunch, it overheated again, so I pop-ed the hood and opened the bleeder... more air. I could hear the impellor blades in the pump engage as water began flowing again, and the temp returned to normal.
So, perhaps the shop fixed the original problem but didn't burp the system. Or perhaps there were multiple problems. I don't know.