Please help me make sense of this log file. Stumped!:

[Mon Jan 21 13:35:57 2013] [error] [client 117.21.225.25] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113, referer: http://www.mbird.com/2009/05/from-one-sufferer-to-another/
[Mon Jan 21 15:28:11 2013] [error] [client 58.22.10.92] File does not exist: /opt/bitnami/apache2/cgi-bin/register.cgi
[Mon Jan 21 16:29:28 2013] [error] [client 184.168.116.156] client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /w00tw00t.at.ISC.SANS.DFind:)
[Mon Jan 21 19:06:33 2013] [error] [client 71.45.112.10] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113, referer: http://www.mbird.com/
[Mon Jan 21 21:22:21 2013] [error] [client 70.178.94.4] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113, referer: http://us.yhs4.search.yahoo.com/yhs/search?p=mockingbird+blog&fr=goodsearch-yhsif

Seems to be related to eventual errors where WordPress reports "Error establishing a database connection"

What else should I investigate? Could this be related to, well, the way I'm calling related posts on single post pages? See: http://pastebin.com/m4f6ekPj

EDIT

Things I've tried:

• Apache tuning
• W3TC with APClink text (APC seemed to go haywire and hog all the memory)
• Removing extra plugins
• CDN

Now I'm on a medium instance and using SuperCache, which should be plenty, but the site still goes down and I see "Error connecting to database". I'm almost ready to quit AWS and bitnami, this has become so frustrating!

Here is today's error logs:

wordpress-error_log:

[Tue Jan 22 02:11:08 2013] [error] [client 54.240.144.137] File does not exist: /opt/bitnami/apps/wordpress1/htdocs/wp-content/uploads/2012/09/BobDylanAtFASNov1975.jpg/QS/462722.jpg
[Tue Jan 22 18:49:16 2013] [error] [client 65.55.217.234] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113
[Tue Jan 22 19:35:55 2013] [error] [client 68.213.119.217] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113
[Tue Jan 22 19:38:03 2013] [error] [client 68.213.119.217] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113
[Tue Jan 22 19:40:05 2013] [error] [client 68.213.119.217] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113
[Tue Jan 22 19:42:09 2013] [error] [client 68.213.119.217] PHP Fatal error:  Call to a member function get() on a non-object in /opt/bitnami/apps/wordpress1/htdocs/wp-includes/cache.php on line 113

error_log:

[Tue Jan 22 15:22:56 2013] [error] server reached MaxClients setting, consider raising the MaxClients setting
[Tue Jan 22 15:51:36 2013] [notice] caught SIGTERM, shutting down
[Tue Jan 22 15:51:55 2013] [notice] Apache/2.2.22 (Unix) PHP/5.3.10 with Suhosin-Patch configured -- resuming normal operations