Sounds like poor contact from the light and batteries. Pulling the spring out a bit could help, but I'd just return it. Mine has many miles of off-roading abuse on a rigid bike, and has not flickered once. My take is that QC isn't great on these, so I'd swap it and hope for a better one.

Do you have the batteries wrapped in paper to eliminate them rattling in the cage? I did this right off, not sure if it would change the flicker issue, but it kills the noise by keeping the batteries tight, and is a very worthwhile 1 minute & free mod IMO.