What type of artifacts? I have heard on this site that if you don't format the cards, and just delete the photos in the camera you are just writing over existing files. Some times this can cause errors and corruption of the picture files. I personaly format the card every time I get all my pics off and I will even format it with the computer (this does not work with all cameras, try a small card you wont miss) and run a disk check program on it.
What you might do is post one of the photos for a sample...