I have the same problem and I contacted Pandora and here is the response they gave me:
This can happen sometimes if the stream is interrupted or has too many lost packets. This can occur on a congested internet connection, for example an office LAN where many people share a single Internet connection.
Also, Pandora uses an audio encoding technique that can confuse certain network security devices if they have out-of-date firmware.
If everyone in your office is experiencing the problem one of these two things could be the cause.
Another scenario is a wifi connection with a weak signal, or even a strong one if there is interference. Interference can come from wireless phones, neighbor's wifi transmitters, baby monitors, microwave ovens etc.
Pandora uses a buffer to prevent most network problems from causing an interruption of the music. Pandora will try to reconnect and continue where it left off. If that fails it will make a new connection and start a new song.
Sorry this has been happening to you. If you are on a wifi connection you might want to check if there are other wifi transmitters nearby with strong signals. If so, you can change yours to a different channel far away from the channel of the other transmitters. Usually channels 1, 6 and 11 are best.