80gb ipod wont hold more than 15gb of music?

I just bought an 80gb 6th gen iPod, but anytime I go over the 15gb mark, iTunes freezes up and will not resume until I unplug the USB (I cannot eject it on "My Devices" or in iTunes, so I have to just unplug without disconnecting it first). I have tried to reset, reboot, and restore the iPod, but it still has the same issue. I have tried adding movie files to the iPod via "My Computer," but it will still freeze up if the total space used is more than 15gb. I have also reinstalled iTunes, deleted and re-imported my music, and added songs by smaller increments, but it still freezes up around the 15gb mark. Any help is greatly appreciated- maybe I need to do some of the things I've already tried, but in a different order?... Hopefully this is not a lost cause, because I really like the iPod.