I just sold two Classics cos they were getting ever more painful to synch. and much less use than they used to.
First of all, I'd create a small playlist and just try to synch that so you're not waiting for ever.
When you've restored the iPod does it show the expected amount of memory free?
Do pics synch OK. Again, put just a few pics in a folder, only synch pics and see if they will synch. It will help isolate it to the iPod or Music.
Assuming you're on a Mac - here are the notes I made the last time I had trouble synching my iPods. They are rambling and I can no longer test them cos I've sold all my Classics. Honestly - I'd wait in case someone comes along who knows what they're doing before deleting plists, although you can always put them back from the bin.
-----rambling starts---
Patience is the best thing. Even if it looks like it's not doing anything, if you wait long enough it'll probably work. If they don't then try delting the plists of
AmpDevicesAgent
com.apple.amp.devicesui (Finder) - no plist
AmpLibraryAgent
AmpArtworkAgent
Deleting the plists of the above (couldn’t find the devices one ) seemed to unlock the Silver iPod when it wouldn’t synch.
---rambling ends----
Good luck.