Page not found error on some post links

I’m just setting up a new WP, and have some posts linked on a page. When you click the link, you get a page not found error for just a few of the posts, but if you go to the links at the bottom of the error page, click the correct category, then the link to the post there it works just fine. It’s only on a few posts, and I can’t see any commonalities. Sometimes deleting the post and redoing it seems to solve the problem and sometimes not.

I’m a bit out of my league with this one. Any ideas? I’m using the default 2011 theme, and wishlist and wpdrip plugins, but I’ve tried turning them all off and still have this problem.

Thanks for any help and advice!
Sandy