Rebuilding thumbnail caches on startup

I'm running the latest version of iPhoto under Lion and it is constantly rebuilding the thumbnail caches when it launches.

It used to do it under Snow Leopard as well.

I have tried holding cmd+option and selected all the various rebuild options, with no change.

I have also trashed the com.apple.iphoto.plist which worked for one restart of the app, but then went straight back to rebuilding the caches.

I have tried with selecting another library by holding option when starting up and that library loads fine.

Any thoughts or assistance would be welcome!

Chris