Wierd bug - items have different thumbnails and fullsize images

After I updated Omeka from 2.2 to 2.5 i discovered a lot of items in a particular collection have the correct thumbnail image but wrong image in the item page (it shows the image of next item). I deleted the collection and import again with csv import. But Problem is still there.

what should i do? Delete Omeka and install everything again?

http://bestand.tws.uni-koeln.de/items/browse?collection=7

i discovered that the Problem occurs when items have the same Titel. Does omeka matches the images with the Items by titel? how can i change it to identifier?

example:
http://bestand.tws.uni-koeln.de/search?query=unbekannte+Personen&query_type=exact_match&record_types[]=Item&record_types[]=File&record_types[]=Collection&submit_search=Suchen

the problem was Clean URL Plugin using title to create the URL. I changed it to identifier, everything is fine now