They changed their search system a few years ago.
I'll tell you what I used to do and what I currently do.
I used to do what they call html hack. When you did a search for threads like 30 days, 1 year, or all time, it would give a certain html link depending on your search(1 month, 3 month, 1 year, all time). I simply would change the html link to whatever search period/year I wanted, and I could do that with specific usernames as well.
People knew I could find old threads because I would bump them a lot, as well as own the fuck out of orcus in our old debates by using his own posts/logic against him. But I actually searched and read far more threads than I actually bumped. If I saw a screen name with a 2001 sign up date(or earlier), especially if they didn't have very many post, I would do an all time search with their screen name to bring up old school threads.
Like if a 2001 member had 300 post, and it was 2015, I would do an all time search under their screen name and find old school threads from the early 2000's. It was fun to read the posts and comments from back then, as I wasn't around that forum at that time and it was also interesting to see how those comments turned out in hindsight.
Anyways, what I would do now if I was you, is try to remember key words in the topic thread title, as well as the thread creator's username and do a google search. Like "mixedmartialarts.com song red panties".
You can't find threads more than a year old unless you use google. They changed it. :/