OK, I am back from a long weekend ... the engine is now working fine: lasted a full flight without quitting. (Idle is not yet low and reliable, but acceptable.)

Here´s what was causing the problem: the glow-plug. ... going back to my first flight where it overheated (melted the plug filament), I had replaced the plug by an OS#8...
This weekend I tried with an A5 glow plug, and voilá...
Also, I removed the cowling to get good cooling airflow.

Thanks again for all your help.