If you take the memory card out of the phone and plug it in to a computer, or plug the phone in to a computer, are the files able to be read correctly? If so, then it seems likely that it's not a memory card problem. Though you might try copying everything from the memory card to a computer (which is something that should really be done periodically anyway). Then use the phone to format the memory card (make sure you're formatting the external storage, not the internal) and then see if things work better.