Try this: launch iPhoto with the Option key held down and create a new, test library. Import some photos and check to see if the same problem persists. If the problem persists in the test library try the following: make a temporary, backup copy (if you don't already have a backup copy) of the library and try the following:
1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your
2 - delete iPhoto's cache file, Cache.db, that is located in your
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3 - launch iPhoto and try again.
NOTE: If you're moved your library from its default location in your Home/Pictures folder you will have to point iPhoto to its new location when you next open iPhoto by holding down the Option key when launching iPhoto. You'll also have to reset the iPhoto's various preferences.
When I started up iPhoto with the option key down the box that appeared above where you select a new library had 6 identically named iPhoto Librarys already listed. I selected one and received a message saying that the Library selected would have to be updated to match the current operating system. I did and it loaded an older version of my iPhotos. I renamed my current preferred library to differentiate it from the others and rechecked the box and it was in last position on the list.
What you might want to do is move all of those libraries into the Pictures folder making sure each has a different name, i.e. iPhoto Library_1, iPhoto Library_2, etc.
Then you can determine which has all of your photos. If you need to merger those libraries in order to get all of your photos you'll need to use iPhoto Library Manager. It will merger the libraries into a new library and cull out any duplicates along the way.