Your case sounds a little unusual since it's moving to feather when stopped. Usually happens due to the very low oil pressure at idle. Most often presents itself with a high OAT and/or an engine which is reaching the end of its life as all the oil leaks past the worn bearings. I suggest keeping RPM at or above around 1000 rpm and usually if you shut down from this it won't feather. Most annoying thing is trying to start the feathered engine and you'll find that a full rich start usually won't work.