The general recommendations:

• use cache plugin for your WP installation
• reduce PHP memory_limit to (32..96M) to start seeing PHP memory exceed errors
• disable useless and new plugins
• make sure all reporting settings are on
• reduce max worker processes by hand (3..10)
• set nonzero MaxRequestsPerChild if you think that PHP/Apache may have leaks (errors in compiled code of PHP interpreter or Apache server)
• reduce ServerLimit
• reduce MaxClients
• use PHP in FPM mode

Specific advices:

• write a bash script to measure memory usage or collect peak and put it in cron. You'll have stats for memory usage of a process over time.
• or use munin as more advanced solution.

The general recommendations:

• use cache plugin for your WP installation
• reduce PHP memory_limit to (32..96M) to start seeing PHP memory exceed errors
• disable useless and new plugins
• make sure all reporting settings are on
• reduce max worker processes by hand (3..10)
• set nonzero MaxRequestsPerChild if you think that PHP/Apache may have leaks (errors in compiled code of PHP interpreter or Apache server)
• reduce ServerLimit
• reduce MaxClients
• use PHP in FPM mode

Specific advices:

• write a bash script to measure memory usage or collect peak and put it in cron. You'll have stats for memory usage of a process over time.
• or use munin as more advanced solution.