There's probably an issue with how fast the camera is writing the JPG preview file that the LCD uses. It's probably slowed right down on the main file as well, you just don't realize it.
I would guess that the memory card's directory is fragmented. The easy way to fix that would be to format the card a few times in camera, and see if it keeps doing it.
Everytime you clear a card, you should format it in camera. That will save you a lot of trouble.
David Buzzard