I FOUND A SOLUTION
i have the exact same problem with my samsung galaxy 3s. the playlists content disapears all the time.
what helped on my phone was to make a backup on your pc, from the phones and SD-cards memory, then format your phone (both phone card) PROBLEM FIXED. I know it takes some time to restore everything, i recommend that you install "backup and restore" and "playlist backup" on your phone and make a backup on your apps and playlist. good luck
PS. install only the apps that you need (it could be an app that did made the bug)
PPS. if you can use another music player than the default one, that could also prevent the bug, i am using "Now Playing"