During peak hours (11:00-6:00 PM CST?) the forum is swamped with activity. This ALWAYS disables the search function. Generally, I find that doing searches before or after this timeframe work fine.
When Parker gets the new server up and running in October the search function will probably work well at any time of day.
When you say it doesn't work is it giving your a message that says its down due to high traffic or is it because it doesn't return the threads that you think should be there?
If its the first option and this is happening at midnight then something is wrong because I have never had it not work at midnight for me.
I delete messages after about 60 days on some forums and longer on others. So if the answer is the second option above it may be that the post has been deleted.