There's no such thing as "manually sync", so perhaps your use of the phrase may be contributing to your problem.
- Manually Manage (to give it its correct name) means that you manually drag songs onto the device, and manually remove them as well.
- With Manually Manage, information about the number of plays, changed star ratings etc. is not sent back to your iTunes Library.
- In addition, Smart Playlists that automatically update themselves will not be updated onto your device.
- Worst of all, if your iPod ever needs to be Restored* (that is completely wiped and start again), you will have to manually drag every single song back onto the device after that Restore.
- Sync means that iTunes will automatically add (and remove, if that's what you want **) songs according to the rules you specify.
If you choose to Sync Entire Music Library, then the first Sync puts every song onto your device - you just get on with something else while iTunes does all the donkey work for you. Subsequent Syncs copy all the changes in your Library, to the device
- Play Count information (the number of times a song has been played) and on-iPod changes to a song's rating are transferred back to your iTunes Library - you do nothing, except let the Sync take place
- If you choose to sync only certain parts of your Library, any changes to those songs, the Playlists they are in and ratings are automatically added to the iPod at the next Sync.
- creation of, or changes to Playlists are then copied onto the Pod by the Sync
- additional songs, added to your Library, are then added to the iPod. If you have a Smart Playlist of music that has been recently added to your iTunes Library, that Playlist will be on the iPod and kept up to date by a Sync
- Syncing only parts of your Library, using Sync is the only way to manage a Library that is too large to fit onto your iPod. **
- When (not if) your iPod crashes and requires a Restore, is lost, stolen, breaks or you simply need to replace it with an newer iPod, one simple Sync will copy you music from your iTunes Library to the new device. Again, you let it do all the work for you while you get on with something else that's far more interesting and far less tedious.
- A Reset will restart the iPod without removing any of you media (songs). A Reset is performed on the iPod Classic without the use of iTunes and can fix many iPod issues
- A Restore (performed using iTunes) may be necessary when a Reset cannot resolve issues. A Restore removes everything from the iPod and puts back on only what you specify from what is in your iTunes Library.
in my opinion, if your iPod Classic has needed a Restore (rather than a Reset) more than once recently, and given its age, that suggests that the iPod may now be struggling to keep working and that it may not be long before it fails completely.
So not so much a bug, but signs of ageing and wear and tear.
One last point: your iPod needs to have a backup of the music on it. That backup should be on your computer and catalogued in your iTunes Library. In turn, you should have a backup of your computer as well.
** Additional comments added by author after initial posting
