I've tried clearing the cashe but it still happens.

It only occurs to the top-most forum, and it only throws in a last post from one particular other forum which also happens to be the most popular one in terms of posts.

This only happens a few times a day. For some reason it'll throw in the last post from another forum and then leave it that way even though the thread (from another forum) will continue to have new posts added.

Of course posting in the first forum "fixes" things temporarily by instantly showing the last post from this forum, but then a few hours later for some mysterious reason it'll show a last post from that different forum...