I meant Sigil the epub editing program (see:
https://code.google.com/p/sigil/) it has a facility to check the code of an epub. The actual tool that does the checking is called flightcrew (
https://code.google.com/p/flightcrew/) and was built into Sigil.
Anyways even with clean books on the device I still get these annoying hang ups. Be reducing the number of books I have on the device I feel it happens less, as well the device seems more responsive IMHO.