I had a similar problem with a large school of Columbian Tetras. Suddenly one of the group would start swimming erratically and even demonstrated a corkscrew type swimming action.
In my case, high nitrates, due to overcrowding, was fueling Some type of internal ailment in that particular species. Upping the WC
's, and thinning down the overall population solved the problem. My guess is your Emperors are suffering periodic internal bacterial infections. Perhaps the above noted preventative measures will help, or maybe the inclusion of occasional feedings of medicated foods.