Try checking the pill in the head and see if it's loose. If so, tightening it up. If that's secure, it could also be the tail switch is loose. Take off the o-ring that secures the tail cap. Then take the cap off. You'll see the actual McClicky switch. See if that's loose and if so tighten it up. If it still flickers then it could be a faulty switch or something else. A faulty switch is easy to replace. You can buy a replacement switch from many different sources but I suppose if it gets down to that, you may just want to contact Don for next steps.