Hey Kirby --
Thanks so much for your detailed response.
Just to fill in a few holes in my story (and answer some of your questions):
The Pro-Kit update is installed.
I have done the repair permissions, but haven't repaired or rebuilt the database (I will try that shortly).
I'm pretty sure I've given Aperture ample time to do its behind the scenes activities, but couldn't hurt to do as you suggest and give it overnight to run.
As far as my photos, I have 20,000. When I switched from iPhoto to Aperture, I elected to reference the iPhoto library (leaving the files where they are). All new photos are being put in Aperture's library. (The problems I am describing above take place when I'm dealing with new (Aperture library) photos; I haven't done much with the iPhoto pictures). My photos are 99% jpg -- haven't experimented with RAW yet. Previews are set to half-size and medium quality (
in Aperture's preferences).
One more thing I just found. In Finder uner User>Photos there are two aperture libraries listed -- Aperture Library and Aperture Trial library. Might that be a problem?
Thanks again for your help. Really want to get this problem fixed....