Seems weird that you would go through them so fast.
I went through my first one really fast, because it was a crappy SD card I got from the Wii powersaves thing (back when I was too stupid to know you can download saves off the internet
), but that was obviously because it was awful quality.
I got a Kingston 2GB SD card and it has been working fine for me so far, not a single problem.
So I'm not sure, but it might just be the quality of the ones you have bought.