>>265
Are you sure you are really seeing the bitrate drop? I don't know how the BBC does things, but it seems more likely they would use a constant bitrate (yeah, or variable bitrate, just not consistently dropping). The "drop" could be because it buffers up data and the player reports a higher initial value.
As for the actual problem, it could be anything; high load on server, home router acting up, etc. If you're listening through a browser, you could try plugging the stream directly into VLC or something and see if that makes any difference. At least it might give a "second opinion" on what's going on.