If the problem began after switching to a different SD card, perhaps the system wasn't completely shutdown when it was powered off?
Linux needs to be shutdown using one of the following commands, or by clicking the Shutdown button if you are using the GUI:
sudo init 0
sudo shutdown -h now
All of the lights except the red power light should go off -- then it is safe to unplug the Pi.
Otherwise, if did this and you still had the problem, then it's probably just a wonky microSD card. I've had new cards start misbehaving on my Beaglebone within a month, while others have never had a problem. I've also found that, once a card reports errors, they generally can't be reused -- after rewriting the image to the card, it will continue to have problems. It's always a good idea to occasionally backup an image of the card to a USB drive, making it less painful to switch to a new card.