OK people -- I don't really know yet what the problem is but I found out how to stop it. (By the way, I hate these lower case letters but in order not to offend some people's delicate sensibilities I'll try to live with them.)
--ON BOARD GLOW PLUG STARTER -- I installed one and now the plane lands with the engine running. Sometime I can hear it cutting out but before the engine stops it catches up again. And I've decided that when it was cutting out it was in level flight at less than full throttle. The starter is adjustable (it reads the throttle position) and it's adjusted so it's on all the time except when the throttle is above 3/4 open.

Ken