It isn't the battery voltage. Mine does this with brand new batteries. Nor do I believe it is the switch, but that is a possibility worth investigating. I do think it might be the fit of the batteries in their holders. Some battery holders are bendable tabs and others are springs. Either way, a firmer, less bouncy contact with the battery must be acquired. I went to Planet Bike flashing arm bands, otherwise I would have found a fix for the blinky by now.