Description:
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After security upgrade from 5.1.4 to 5.1.6 on the production server, we have performance problems. CPU load was about 0.6 - 1 with version 5.1.4, but after ugprade to 5.1.6, load is from 1.5 - 3 and Apache somtimes starts forking too many childs. If we downgrade back to 5.1.4, load goes back to 0.6 - 1.

Reproduce code:
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FreeBSD 6.0-STABLE SMP (Dual Xeon 3GHz with HTT + 2GB RAM)
Apache 2.0.59
PHP5 5.1.6

Installed PHP extensions:
php5-5.1.6
php5-bz2-5.1.6
php5-curl-5.1.6
php5-dom-5.1.6
php5-exif-5.1.6
php5-extensions-1.0
php5-ftp-5.1.6
php5-gd-5.1.6
php5-iconv-5.1.6
php5-mbstring-5.1.6
php5-mhash-5.1.6
php5-mysql-5.1.6
php5-mysqli-5.1.6
php5-openssl-5.1.6
php5-pcre-5.1.6
php5-posix-5.1.6
php5-session-5.1.6
php5-snmp-5.1.6
php5-sockets-5.1.6
php5-sysvmsg-5.1.6
php5-sysvsem-5.1.6
php5-sysvshm-5.1.6
php5-xml-5.1.6
php5-xmlreader-5.1.6
php5-xmlrpc-5.1.6
php5-xmlwriter-5.1.6
php5-zlib-5.1.6
pecl-PDO-1.0.3
pecl-PDO_MYSQL-1.0.1
pecl-zip-1.7.0

Expected result:
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I expect same performance as with 5.1.4

Actual result:
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The result is that webaplication (custom CMS) became unresponsive under heavy load. Normaly Apache runs with about 20 childs, but from time to time (1 - 10 times per day) Apache starts forking new childs until MaxClients limit is reached, then is restarted by "watchdog script".
There was not this type of problem until upgrade to 5.1.6 
2 same servers are running only Apache and one virtual host (loadbalanced) with one webapplication, database is on the third machine, so there is no "hidden" things causing this load.

Let me know if I can do/post more to find source of this performance problem.