Corrupt song?
I had this problem before Lion. It was related to a corrupt song file--even if I didn't play it on my iPod, it froze it. I sorted by file size and found one (from a public domain collection of old 30s music) that was obviously too small for the music length. It may not be that easy for you to track down, though, so you may have to keep manually syncing until you add a playlist with the offending file and can track it down.