I've seen that issue but *not* on PB360.
Where the files ever DRM'ed? Any idea how they were made?
(When in doubt, blame DRM. ADE is also a good scapegoat but not this time.
)
If they are calibre conversions to ePub, try converting with different settings or a different target. For fun, try converting to mobi, lit, or fb2 and back to ePub.
(Thing is, Calibre has a strong ADE-bias; whenever there is an issue it gets recoded to output files that display properly in ADE. Other apps that stick to the spec find themselves at a disadvantage.)