Hi, just found this after Googling the same problem. I hope this will help other Audible.com users who like long biographies and the like, but can't play them to the end on their iPods.
My iPod Nano is a gen 7, and either starts stuttering like a stuck LP record or jumps back to page one of the audiobook after flawlessly playing a good part of a long book. I suppose it must be about 24 hours into it, as it didn't falter on a 23+ hour book. My laptop and my Kindle would play the books all the way to the end, so it wasn't a problem with the files themselves. The local Apple store tried reinstalling the iPod's firmware, to no avail, so they gave me a new iPod. I had the same problem with the new device.
Today I had a talk with a supervisor in the Apple support group who was sympathetic and said that she would refer the issue to their engineeriing staff and get back to me. In the meantime, I talked with Audible.com, and the support guy there solved the problem for me. All you have to do is sign in to your Audible account, go to your Library, and find the "Audio Quality" button at the upper right corner. Mine was on "Enhanced", and he had me switch it to "Format 4". That resulted in all the audiobooks in the library to be displayed as both a single file and as multiple files (Part 1, 2, 3, etc.). I was able to download the final parts and successfully listened to them on my iPod.
Nonetheless, this does seem like an issue that Apple would want to address with a software enhancement.