Hello and thank you for your question. I look forward to assisting you today. Are you powering them by the battery when this happens?
So just to make sure I understand, you are strictly using the battery and no wall power when this happens?
Have you noticed if they are getting unusually warm?
Okay. You have done all of the troubleshooting steps I would have taken. Have you checked the auto power off to make sure it didn't get turned on somehow?
This sounds like a problem with the main circuit board. The only fix is to send it in for service either back to Canon, or I can recommend another service center if you like?