You can try the Reset Settings command. This command is on the iPod's onboard Settings screen. It should be at the bottom of the list. This resets all settings on the iPod to default, in case a corrupted setting is causing the problem.
Another thing to try is to remove all audiobooks from the iPod, by changing the iPod's automatic sync setting (in iTunes) to NOT sync any audiobooks (turn off syncing for audiobooks). Sync iPod to remove audiobooks. Then, set up syncing for audiobooks again, like it was before. Sync iPod to add audiobooks. See if there is any difference with fresh copies of the audiobook files on iPod. NOTE: If you are using the manual setting to load audiobooks, the process to remove audiobooks from iPod is different; you can post back for more info.
If that does not help, you may want to do a Restore on iPod using iTunes. This erases iPod, reinstalls its software, and sets it to default settings.