hmm, ive had same problem with my 3g 40gb for some time, on my existing ppc lcd imac...
(ipod locks up while i sync, had to somehow kill itunes as it also locks)
ive suspected two things...
a) bad hardware on ipod
b) something incomatible on newer itunes/osx
c) something incompatible because i also started syncing a 4g ipod, and that about when problems started
the only way ive got around it is to reinstall itunes, and factory reset ipod, then reupdate ipod.
then i can usually reinstall all tunes (about 20gb worth)
BUT (heres the bad news), some time later, the problem just reappears.
unfortunately, ive google'd lots, and can find noone else complaining of it, so i put it down to bad hardware ... even though this seems unlikely, as ipod is fine, except when i go to sync!!!