@R1D2  Thanks for your input.  Oddly dropping the unit out of the plastic mounting plate appears to resolve the issue even with the original cable.  (Switching ports did not help.)  Because that the socket is at right angles to the hole through the ceiling, the cable has to go through quite a sharp angle once it enters the access point recess .  It might well be that the thick and faily inflexible cable that I am using with some beginners crimping is causing the problem.  Anyway, at least I have an avenue for further investigation.