It sounds like there is possibly something wrong with your flashlight and I can understand why you would assume it is PWM if it is exhibiting that behavior?!?! Feel free to send it in to me for evaluation and possible converter replacement.
EDIT: there have been some odd cases in the past when R123's are powering the light. Some of them have their own circuits which might bepart of the issue but heck if I know what is going on. Does the light behave in the same fashion if you use a primary?