Summary to date: for a month or three the car would die quietly at stop lights. In the last week the car started to run very badly and throw a code.
Fixed: the latest problem. I noticed coolant circulation problems and got suspicious of the radiator cap, replaced it. Now the car doesn't have its fits of running very badly.
Still to go: car dies quietly at stop lights, very occasionally. It appears this happens after five to ten minutes of running steadily at a good speed (40 MPH or better) followed by a stop. Car will also display "check engine" light out of the blue *without* stalling, code reader gives "MAP sensor output voltage too high".