iPhone occasionally plays the wrong song in untagged playlists

I'm putting a lot of music on my iPhone, and I'm having a strange problem. A lot of this music is not ID3 tagged, and cannot easily be tagged since it's custom mixes, home-made Greatest Hits albums, that kind of thing. Some of it was converted from tapes, records, etc. where the original material has never been available on CD. This music is coming from my music library on my computer, where to ease the generation of playlists, I made the decision more than ten years ago to call individual files letters of the alphabet, based on their order in the playlist, E.G. a.mp3, b.mp3, c.mp3, etc. I have well over 2000 such playlists on the computer, so changing at this time would be difficult.

Sometimes when I put a playlist of unidentified material in iTunes and sync, when I then play that playlist, it will play one totally unrelated song instead of one I want. For instance, in one of my playlists, for the file H, it plays a totally unrelated H file rather than the one that's supposed to be there. This seems to happen, again, with material with no ID3 tags.

Has anyone else experienced this, or do you have any thoughts or suggestions of how to fix it?

Thanks.