It could have something to do with the 'contention ratio' on your broadband connection - if you have a 1 meg connection with a 20:1 ratio, and there are twenty of you sharing (which is what it means), then you only get a twentieth of your rated speed...

Dial-up is generally too slow for RealPlayer streaming anyway.

It could also be because you're trying to listen to a popular programme, causing the Beeb's server connection to slow down.

It could also be your RealPlayer buffering settings... it's been a while since I fiddled, so I can't just remember where they are... I'll have a look when I get back from lunch (unless someone else beats me to it)!