First of all , thank you very much for all of the suggestions !
So , as it happens , I did the old paperclip trick , and sure enough , the light functioned properly .
After examining the switch , I determined that it wasn't making proper contact . So I put a solder blob on the edge of the contact ring and that fixed the problem !
Since the iron was still hot , I replaced the stock driver with a 3.4 amp Qlite .
When the problem reoccurred , I checked every connection and everything else I could think of .
Then I tried another battery in the light .
Well I be damned . The nearly new NCR18650A I had been using was causing the problem !
It was down to 2.34 volts . I never run my batteries down past 3 volts so something had gone wrong with that particular battery . It hadn't even been recharged more than two dozen times .