iPod not syncing properly on iMac

Hello,

I am trying to upload all of my music to my iPod classic (almost 20k songs in all).

When I do, it will start syncing just fine but then start to say that certain songs cannot be loaded because they are not supported and then stop the whole process. So I just deleted the song and continued and it would continue to sync remaining songs, and then another song will come up that' not supported. So I had to keep deleting the individual songs and then continue to sync. But there are a couple of things strange about this.

1 - The songs that are "not supported" are very random. It's not songs I have purchased. They are songs that have been uploaded from a CD, and even if the rest of the album syncs properly, on certain songs are "not supported" which doesn't make any sense.

2 - The last time I was told that there was a file that was not supported, I deleted it. But when I slicked "sync", everything was cleared from my iPod.
I already tried restoring to my original iPod settings through iTunes and I reformatted with Disk utility and it still start syncing again, but then I keep getting the "not supported" thing.

Since it seems to still be able to sync thousands of songs, I don't think this is a problem with my iPod. I have the latest version of iTunes, so I am wondering what it could be? Any guesses? thanks!

Usually when you add music from a commercial CD or DVD that you own, it syncs properly to iPods. Unless there is some sort of DRM involved, you should be able to sync everything. How old are the CDs? Do the songs that won't sync play properly when you insert the CD? I sync home made recordings to my iPods and occasionally run into difficulty with certain recordings, and it's usually due to a recording error or bad spot on the CD.