Hackers inserting post

Every few days my site gets 10 pages inserted about cars. Nissan, Volvo, etc. The post have no external links but all pages link together.

The pages do not appear in the back end of my website and can only be found using the search feature on my site. I can only see them in the backend when I use the “link to a page” function within a post.

I have done a lot of searching in forums and have not come up with a solution. I have changed all passwords to my users and databases. My database, WordPress and plugins are all up to date.

At one point I deleted a few files that appeared to be random in my website root which fixed the volume of articles that were being published (at the time it was 50-60 per day) but cannot pin point where the injection is happening.

My work around is to go into my DB every few days and just delete the rows but its driving me nuts.

anyone else have this problem?
Image of rows from my DB