This is my response from Canon about the above issue.
"[i]Thank you for contacting Canon product support. We value you as a Canon customer and appreciate the opportunity to assist you. I am sorry to hear that your EOS 5D Mark III cameras are locking up intermittently.We are not aware that there is an issue with these Sandisk cards within the camera. If you suspect it to be the cards, I would recommend trying a different brand and or size to see if the issues persist. We are not aware in advance when new firmware and or changes are coming to the camera. Anything like this would be announced on the Canon EOS web site. I would say that if another card causes the same issue you would want to send the camera in for service. Currently we are not able to recommend one specific brand of cards over another. We apologize for any inconvenience that this may cause you at this time.I hope this information is helpful to you. Please let us know if we can be of any further assistance with your EOS 5D Mark III.Thank you for choosing Canon.Sincerely,[/i][i]TimothyTechnical Support Representative"[/i]