Random iPhoto 11 images missing

Question

Hi there - I'm new to the community and had a couple of questions regarding iPhoto '11 (which continues to frustrate the heck out of me!).
1. Some of my images that were in my iPhoto library are now showing only the cached thumbnails.  When I click on the thumbnail, no image is displayed, just a big explanation mark.  If I select 'Reveal in Finder', none of the options (Original/Modified) are available.  I am able to manually locate the image on my drive however and can confirm that the location and filename have not been changed.
2. I'll sometimes get the 'photo cannot be displayed because the file could not be found' dialog, even though I was able to view the image without a problem the previous time I had opened iPhoto.  I go through the dialog steps of re-locating the photo (which had never moved in the first place) and all seems well but after a few times opening iPhoto, I'll start to get the same dialog again.
This same issue has happened several times since I first installed iPhoto when it was first released. I've rebuilt the library & thumbnails, re-located 'missing' files and even started from scratch a couple of times.  iPhoto seems to play well for a time, then these issues start back up again.  I'd like to keep using iPhoto but I'm seriously frustrated; any help would be appreciated!
Not sure if it needs mentioning but I only have around 4000 images and don't use the 'Copy items to the iPhoto library' preference - I prefer to have my images outside iPhoto because of all the hassles I've been having.
Cheers!

Answer 1

The exclamtation mark is an indication that the file path to the original file has been broken.
Since you are running a "referecned" library you will have to manually relink the thumbnails to the original files wherever they may be.  You can do that with those that give your the "Can't find file" messager. 
So try this: make a temporary, backup copy (if you don't already have a backup copy) of the library and apply the two fixes below in order as needed:
Fix #1
Launch iPhoto with the Command+Option keys held down and rebuild the library.
Select the options identified in the screenshot. 
Fix #2
Using iPhoto Library Manager  to Rebuild Your iPhoto Library
Download iPhoto Library Manager and launch.
Click on the Add Library button, navigate to your Home/Pictures folder and select your iPhoto Library folder.
Now that the library is listed in the left hand pane of iPLM, click on your library and go to the File ➙ Rebuild Library menu option
In the next  window name the new library and select the location you want it to be placed.
Click on the Create button.
Note: This creates a new library based on the LIbraryData.xml file in the library and will recover Events, Albums, keywords, titles and comments but not books, calendars or slideshows. The original library will be left untouched for further attempts at fixing the problem or in case the rebuilt library is not satisfactory.
You probably will continue to have problems with the library seeing as it's a "referenced" library and is very fragile. 
OT