Thanks to replies from Donnie, Benzmac and Maurice dePiero, the following were suggested causes:

1 - Faulty coolant sensor (4 pole on the 1990 CA 560SEL) - I checked the impedance against the chart in the MB manual, and it was OK but at only $12 I replaced it anyway. You need to remove the filter cover to gain access - it sits right behind the cold start valve and requires a 22mm deep socket for removal. Still stalls.

2 - Sticking idle air control valve - that's the 4" long cylinder just in front of the air filter cover with a 2 pole electrical contact and two large rubber air tubes connected. I removed mine and checked for easy sliding of the piston - you can hear it when you tilt the valve. Mine was fine but I cleaned it with carb cleaner just in case. Still stalls.

3 - Removed and replaced the fuel pump relay (same part re-used) after cleaning contacts - it sits on the fire wall on the driver's side, just to the left (facing engine) of the red over-voltage protection relay with its two 10 amp fuses under a clear cover. Stall cured....so far.

Thanks to everyone for their help.