thanks for the replies. these are .mobi 'backups'. drag & drop did not work, so i started using calibre which ive at least had some success with. you may be right, that it is one file that is corrupting it - so i'll try using a smaller set of them for now.
check out this bizarre thread i found, i am definitely not alone. i just tried the 'fix' and sadly it didnt solve the problem outright.
http://www.amazon.com/forum/kindle?_...x3C318LNMKG05B