When you start the slideshow are any of the slides selected or just the album in the left hand pane?
Try the following:
1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete iPhoto's cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iPhoto folder (Snow Leopard and Earlier).
or with Lion, Mt. Lion or Mavericks delete the contents the User/Library/Containers/com.apple.iPhoto/
Data/Library/Caches/com.apple.iPhoto folder.
3 - reboot, launch iPhoto and try the slideshow again.
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and press the Return key - 10.7: Un-hide the User Library folder.
If you're running Mavericks, 10.9, go to your Home folder and use the View ➙ Show View Options menu to bring the this window:
where you can check the Show Library Folder checkbox.
OT