So... what's fundamentally the problem? (excluding the fact the forum has been abused previously).
That if we use the regular search it'll crash the forums every 20 odd seconds, as experienced previously.
We've tried cleaning up the forum, as well as adjusting the search and several other things but the SMF search is just not able to handle the large size and abuse this forum has experienced.
Hence why we've added google search, which only works for public searches (so any post a 'guest' can see.)