404 errors

I don’t know what is going on and I’m going cross eyed trying to figure it out. While most of the site is working beautifully, it is throwing 404 pages very often. The frustrating part is that the article is in the database, the picture and intro text is displayed in the category on the site, but if you click the link itself to go to the article, the site gives a 404 page. Toughest thing is too that it isn’t all the articles, only some, and in different categories too. Have no idea where to start looking. can you help me please?