Could be a slow card, or a card with a bad memory location. To fix bad memory locations, a low level format needs to be done. Do it in your computer for a CF card, or in Camera for a SDHC / XC card
Why would you not format a CF card in camera?
With CF cards, the camera does not actually format the card, just changes the fat table to tell the camera the card is empty. If the card has a bad memory location, you want to do the type of format that actually erases or tests each memory location, and marks blocks with a bad memory location as unusable. This is often referred to as a low level format.
For the 5D MK III, there is a option with SD cards to do a low level or normal format in camera, but not CF.
Lots of people format in camera and do not understand that that type of format will not detect or fix bad memory locations. Its fine to format in camera, but if you are having problems, a low level format is one of the things to try.