If you need to go farther back, you will need to you a regular search engine with a site-delimited search string. Try this search string that limits the search to a username, year, and Apple Support Communities:
Username +"username”+"2010" site:discussions.apple.com
Replace "username" that is in quotes with yours, but keep the quotes. If you do not need a date search, leave that part out. You can add a subject in the same way the year is added in the example:
Username +"username”+"reset NRVAM"+"2010" site:discussions.apple.com
This is one of the busiest forums on the internet with millions of users worldwide. I believe the sheer volume of posts stored is why we see the limitation on in-forums searches.