Does this happen no matter what USB port you plug the iPod into? Have you tried using a different USB sync cable to see if that makes any difference?
Sounds like there may be issues with your iPod's hard drive. Perhaps it would be worth it to run a disk diagnostic on the iPod's HD to make sure it is still in a healthy condition. See this older post from another forum member turingtest2 on how to perform this test and what the resulting numbers indicate.