Thanks for the advice, and apologies for not getting back to this thread sooner, but this seems to have gotten a lot stranger than it looked at first. The computer in question isn't mine, I just keep fixing it when this happens. The 3 hour time limit seems to have been resolved by the user changing both the playlist name in WMP and the filename to match each other. Unfortunately this might just have been a symptom of something bigger. For a long time now, I've had to go and fix this particular music collection because of things happening to it. Sometimes large portions of it are deleted, renamed, or duplicated. Sometimes the names of .mp3 files are changed, breaking the playlists, sometimes the playlists are renamed or changed in some way. To try and fix this, I've turned off all options I could find that automatically rename or move music files.
I've just had a call to say that a large portion of the library has been duplicated, apparently there are now 2 copies of the files themselves. At the same time, a lot of the entries in the playlists are now broken and won't play. This is on a completely new computer running a different version of windows, the machine this started on was a windows 10 tablet, now the problem has apparently moved to the windows 8.1 desktop which has another copy of the same music library. I don't understand what keeps causing this to happen, there seems to be no pattern to it, the files just won't stay still.