When using a micro SD card, it will fragment over time no matter how much storage it has. This means that you can have no files on your SD card and yet this fragmenting will interrupt the high transfer rates required to film in such high resolutions.
To fix this problem, you can reformat your SD card, giving you a completely blank SD card to work with. This should allow you to film in high resolutions. To reformat your SD card, delete all the files off of it, and reinsert it into your GoPro. This should fix any problems you have. For more information regarding this problem, see the link below.
GoPro Hero3+ Silver Edition Troubleshooting