Hey Hoai Anh,
I would try backing up and restoring your iPod. Just make sure that, before restoring, you back up the iPod and any files saved on the iPod's disk. You can read about how to restore here:
Warning: Because restoring erases all the songs and files on iPod, back up any files you've saved on the iPod disk. All of your songs, videos, podcasts, audiobooks, and games can be loaded back to your iPod provided that you have them stored in your iTunes Library. If you use the Nike + iPod Sport Kit, see this document for more information.
Have a good one,