If this began happening when you added a certain book, you might take that book off and see what happens.
Another thing to try would be to update or reinstall the firmware. Even if you have the current one, install it again in case it got a glitch. Use the manual method–the updater is useless since this is the last update.
http://forums.sandisk.com/t5/Fuze/Sansa-Fuze-Firmware-Update-01-02-31-amp-02-03-33/td-p/139175
