Edit Posting Problem...
 

Everytime someone edits a post within a thread, their post is placed at the top of the thread (first) ... eventhough they aren't the author of that thread.

How can I set this back to placing the original thread starter as the first poster inside this thread?

hmm, i'd say you've installed a hack which changes the date of the post, and because of posts are sorted by the date, you have to edit the postdate again, and als correct the bug in your editpost.php :)

could you take a look at my editpost.php file?

send it to me per pm, i'll take a short look at it :)