You should be fine. Canon recommends using at least 8 MB/s cards. But, as Daniel stated, what a manufacturer states as a speed and what the speed is in practice depend on the card and your camera. Rob Galbraith has tested many card types in several cameras. Unfortunately, the 7D is not one of the cameras he tested. But, to apply some of his numbers, your Sandisk Extreme III 30 MB/s card tests out with an actual write speed of 18.4 MB/s on a 50D, but only 7.5 MB/s on a 1DIII.


So, it could still be the card, or the camera itself, or the software you're using. Perhaps try a different viewer ( Canon's ZoomBrowser, for example, or some other free software). If the problem persists, perhaps getting a new card is the answer. Also, make sure you're running the latest firmware (1.2.2).