I was watching Fro Know Photo the other day, and he had a "speedlight expert" guy on. They did a long, like 45 minute video, and the speedlight flash expert mentioned that some battery brands "run hot", and that one of the Nikon speedlights did not like to work well on brands that "run hot".
What brand and milliampere-hour batteries have you been using? Are you using all batteries of the same brand and specification at one time? What "kind" of batteries have you been using, meaning the type of material in them--lithium? alkaline? Ni-Cad?