Sorry folks. I realized that I never gave the full run down of what I was using...
I have a MacBook Pro (2.4 ghz) from around late 2007/early 2008. I am running 10.5.8, and have the sony PRS-300.
The suggestion to remove all components of the install - and then reinstall the Sony software from the web download (presumably the most updated version available), is working so far.
Keep your fingers crossed!
Thanks again for all the help.