This problem is very annoying.
In my case the problem is identified with the card reader of cybook (not the SD card). Hardware problem with the card reader of the Cybook.
I bought many SD cards, and all had the same problem. The problem is that the Cybook (with SD card) when the PC is connected to the reader of Cybook corrupts the files of the card.
Even transferring files directly to the card (without using the Cybook) with a time Cybook will corrupt the files.
Look I'm speaking only in my case.
The solution was found using only the internal memory of the Cybook (without using SD card). 4 months have passed without any problem of screen freeze.
forgiveness for my poor English.