I have had this exact problem on my Galaxy s2 since upgrading to 4.1.2. I have discovered that moving the custom ringtones and notifications to the phone's memory rather than the sd card takes care of the disappearing ringtones. So, moving all your music to the phone may or may not solve the playlist problem. However, as in my situation, this is probably not possible or desired due to the amount of memory it would require. So, my solution has been to download the free app called Playlist Backup. It backs up your playlists on the phone and makes it easy to get them back when the disappear. In fact, I just had to use it this morning. This time my playlists lasted about 5 days before suddenly disappearing. I don't know if the battery level is related. It only took about 30 seconds to import all of my backed up playlists to the music player. Obviously there's still a bug, but at least this allows me to spend the time making playlists knowing that I won't have to constantly start over a few days later. Hope this helps!