Listing results showing page not found…

Hi, when searching for listings they come up fine in results but then when you click to go through to the actual page is shows the page is not available ‘Not found, error 404’- any idea why this may be. It is ones that have not been amended, nor expired and i find re-publishing the listings sorts this out (but only if i know they are like it in the first place!), although sometimes i have to change the permalink to get it to work properly. There is nothing in the admin pages to indicate any issue with such listings, it is not all of them, and there have been a couple I’ve come across like it over the last few weeks- any idea what may be causing this?