I had that problem with the GoPro 3, applied the latest firmware and all was fixed. Find it hard to believe it's the media and not the camera.
It is definately an issue with the Sandisk cards as people using the Samsung Pro were not having the same issues.
While your problem was fixed with the firmware update, all GoPro did was to automatically lower the data rate from 45Mbps to 35Mbps for the Sandisk cards (and others that were no supported). Record some footage and then view the file properties, it will probably say 35Mbps if you are using Sandisk cards. If so, you're not getting the full capability of the camera.
With my Hero3+ Black and a Samsung Pro card I get 45Mbps.