Search - why do i have to wait between searches??

Sometimes i wait and sometimes i dont

such a strange restriction

The forum is set up like that in order to counteract post flooders, spammers etc.

and because of that some forums also have the restriction that you have to wait (up to 2 minutes) between two posts.
Also have in mind that doing a search produces traffic on the server. Sometimes you see an error-message if too many searches have to be processed: you are not the only one who is looking for porn