using iPhoto 11 9.4.2 database continually needs to rebuild for inconsistencies

I am using iPhoto 11 version 9.4.2 on a macbook pro os 10.7.5. I often get messages indicating that there are inconsistances in the database and a rebuild is necessary. In between the times when this occurs I have done little work in the application. Also the application seems to take longer to open than in the past. I have a little of 3000 photos.