B. Diddy is right, it's either a bad card or a bad file on the card. Only way to know is to either test the card as suggested or back all of it's contents up, format it and slowly copy files onto it in batches until the problem reappears. If it were me I would pull the contents of the card off onto a hard drive using a computer, then format the card and check it with chkdsk (assuming windows PC) verifying free space as well.
If scans come up clean I'd then copy one folder at a time back onto the card and test the phone as I go. The trouble with doing this is figuring out what file is corrupt . . I've heard of people loading thousands of music files or other types of media files and having a bear of a time finding that ONE single corrupt file . . .
Good luck!