I don't know if this will help, but you might install
z4root from the Amazon Appstore. I don't know if there is any rhyme or reason to it, but several attempted app installs from Amazon failed on me before I rooted my PE. I have had far fewer of these since I rooted. (Coincidence? Maaaaayyyybe...)
edit: This is why I am usually loath to spend MONEY on an Amazon app. I have bought a couple that failed to install, no matter how many rain-dances I did.