Apache first request slow

[davabe]

I have a server configured with default plesk 9.5 options and 1 site with joomla and always the page generation/time request on apachebench (and in web-browsing) was stable (around 0.5sec). One day I have a big plesk update problem and my server crashed. I have reinstalled the server with the default plesk 9.5 configuration. Now, the same configuration, same joomla, same environment throws a horrible timmings at first page generation/request. No matter I do this always happens, with dso/fcgi, with/without cache, other scripts/cms, dns lookup on/off, keep alive on/off, memcache on/off, eaccelerator/apc/xcache on/off...

server specs = centos 5.5 x64, 8gb ram, i5 750 x4 @2,67Ghz, SSD 80GB

server load= 0.00 0.01 0.01



TEST FROM SERVER
After reinstallation: several times "ab -n1 h t t p:// mysite .com/"

Time per request: 7021.168 [ms]
Time per request: 1141.502 [ms]
Time per request: 271.120 [ms]
Time per request: 262.148 [ms]
Time per request: 270.364 [ms]
Time per request: 281.762 [ms]
Time per request: 265.561 [ms]

This first slow request happens every 180 sec, if I wait 180 sec from my last visit or apachebench the request is so slow, no matter if localhost, internet, another domain, ecc..

Before reinstallation: several times "ab -n1 h t t p:// mysite .com/"

Time per request: 521.221 [ms]
Time per request: 643.121 [ms]
Time per request: 583.772 [ms]
Time per request: 599.185 [ms]
Time per request: 541.441 [ms]
...

I was investigating if it was PHP but it's clearly apache.

From shell:

php test.php

call sqrt() 3,000,000 times will cost 0.477s

ab -n1 http://domain/test.php

Time per request: 2315.671 [ms]
Time per request: 642.123 [ms]
Time per request: 616.456 [ms]

A simple html or php file with <?php phpinfo(); ?>
Code:

Time per request: 1013.157 [ms]
Time per request: 71.378 [ms]
Time per request: 62.541 [ms]

apachebench and web browsing shows me the same, the first and sometimes second request is slow, but subsequent requests pretty fast. can you help me?