My 30GB iPod seems to have a problem working with Audible's sound files. On one downloaded book, after transferring the file to the iPod using EphPod (Windows-based software), it will work flawlessly on the iPod for some time (play, rewind, etc. work). Perhaps 1 day of normal uses passes, with the iPod being turned off and on. Then, when I try to access the file, I cannot play it. I can only get as far as the Browse>Albums>Audible.com>track title listing in iPod. Pressing enter usually brings one to the "Now Playing" menu; instead, nothing happens.
The only thing that helps is deleting the file on the iPod and then downloaded the exact same file from my computer (using EphPod). At this point it will work normally, for a while, and then stops as described above.

I'd appreciate any help with this problem. It seems obvious there is no problem with the raw Audible .aa file itself - as when I delete and redownload to the iPod from my computer it works once again for a while.

Does anyone know of any glitches between iPod and the Audible format?

-30GB iPod
-using ONLY Audible format 4 (I've heard others here having problems with iPod and Audible format 2, but I'm not using that)
-EphPod to transfer Audible.com files

Thanks,
Mike