1. Have you tried reseting your player by pressing and holding Menu + Select?
2. Does this happen on all MP3s or just one particular track? If it happens on just one track, then the problem is likely that track.
3. Do you have any other information that you can provide? For example, does this happen as soon as you turn on the player, or after a certain amount of time? Is there anything else that you are doing while listening to music that may be causing a problem (for example, playing a game)?