This happens to me too. I’m listening from my Roku. After a couple hours it goes to the next tract but the sound is gone. It looks like it’s playing the next song but it just goes silent. I am using the free service and I thought about subscribing but it sounds like the paid subscribers have the same problem.
Finally, try deleting the Pandora channel from My Channels, and try re-adding it from the Channel Store. (You can find the Remove channel option by pressing the * key on your remote while the Pandora channel is highlighted on the home screen.)