What version of iPhoto are you using? Have you tried another slideshow? Have you tried changing the order of the slides?
As a test launch iPhoto with the Option key held down and create a new, test library. Import some photos, create a slideshow and test to see if the same problem persists. Does it?
Apply the two fixes below in order as needed:
1 - launch iPhoto with the Command+Option keys held down and rebuild the library.
2 - run Option #4 to rebuild the database followed by Option #1 to reapair permissions if #4 doesn't help.
Using iPhoto Library Manager to Rebuild Your iPhoto Library
1 - download iPhoto Library Manager and launch.
2 - click on the Add Library button and select the library you want to add in the selection window..
3 - Now that the library is listed in the left hand pane of iPLM, click on your library and go to the Library ➙ Rebuild Library menu option.
4 - In the next window name the new library and select the location you want it to be placed.
5 - 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.
If still not working log into another user account on your Mac and try a slideshow from there. If it works there then the problem is limited to your account. If it fails there it's system wide and may require a reinstall of iPhoto or the system.