m250 auto power off

My Sansa m250 is very inconsistent in the time it plays before automatically turning off. At times, it will go the length of time set in the sleep timer. But then often will play for only about 10-15 seconds before shutting itself off. It seems to go in streaks of 5-10 times of shutting itself off quickly, within seconds or a few minutes, then days of playing for the set time limit. The settings are set the same during both of these periods of premature and appropriate shutoff. Batteries are always adequate. Anyone have any suggestions?