Major repeating PHP error: Maximum execution time of 30 seconds exceeded

Hello,

My site occasionally returns huge, HUGE numbers of this error over and over again:

2014/03/06 13:07:37 [error] 16101#0: *3413949 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /usr/share/nginx/www/wp-includes/cache.php on line 456
PHP message: PHP Warning:  reset() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 424
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PHP message: PHP Warning:  current() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 427
PHP message: PHP Warning:  next() expects parameter 1 to be array, boolean given in /usr/share/nginx/www/wp-includes/plugin.php on line 431
PH

I believe the speed and quantity of this error are bringing down the server intermittently.

Here is an excerpt of the lines mentioned in the error:

// Sort
	if ( !isset( $merged_filters[ $tag ] ) ) {
		ksort($wp_filter[$tag]);
		$merged_filters[ $tag ] = true;
	}

	reset( $wp_filter[ $tag ] );

	do {
		foreach ( (array) current($wp_filter[$tag]) as $the_ )
			if ( !is_null($the_['function']) )
				call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));

	} while ( next($wp_filter[$tag]) !== false );

	array_pop($wp_current_filter);

Where should I dig to find out what's causing this?

Thanks,
Paul