Were there video screens or a large led display behind the act? Sounds to me like a clearscan issue, where the camera is out of phase with powered source of light. This happens most often with computer or tv displays, but can also happen with certain type of lights. Traditional video cameras above a certain level typically have a clearscan mode so you can change the hz of the electronic shutter. On a 5d3 you have limited options. Especially in a concert situation. Running the camera off mains at the same phase is one fix where practical, sometimes juggling the shutter can help. Post a clip, but it doesn't sound like a fault.