As you mentioned in your ticket that this is only happening when using the web player, the current assumption is that your web player is performing a disconnect/reconnect every 20 minutes, in order to prevent the player from using up all RAM memory (which can happen when using a Flash player in conjunction with a continuous stream, such as an Icecast/Shoutcast stream), which in turn is causing the introduction file to play every time the Flash player reconnects.
If anyone thinks it might be from something else, or knows of any solution, feel free to contribute to this topic.