What is the brand of the card? If its a Transcend card that could be your problem. We got a Transcend (16gb "so called class 10") SD card with the D3100 we bought from Amazon last January. Before buying more Transcend cards I did a search to find out more about Transcend and what I found out was pretty awful.
One of the people blogging about their problems with that card had bought a bunch of them because he was going on "a once in a life time trip around the world". His Transcend cards started failing half way through his trip. Since he had "once in a lifetime" photos on them he paid a data recovery service big bucks to recover "most" of his photos - regular data recovery software didn't work.
Also I trust that you are reformatting your cards in the camera, letting your computer reformat can cause problems too. But from what you are saying I'm reminded of the horror stories I read from folks who had bad experiences with Transcend memory cards.