Vbulletin Msg Board

1) From time to time my vbulleting message board loads slow, and at that time the server isn't even at a high load this happen at loads 0.9 nothing much really but always below 4.

2) Not only does it load slow but it often return the server cannot be found error, this minute it works fine and the next minute it doesn't, all i can see from the error log for my domain is

3) php warning: Zend Optimizer for PHP 4.3.x cannot be found (expected at '/usr/local/Zend/lib/Optimizer-2.1.0/php-4.3.x/Zendoptimizer.so') - try reinstalling the zend optimizer in Unknown on line 0, referer [url]

4) Does vbulletin relies on zend at all to be fully functional?

5) will zend actualy help at all?

Yes there are other sites on the box and they seem to work fine.
rhel4, ensim X 10.1 with all hot fix to date.
php4
mysql4