Allowed memory size of 134217728 bytes exhausted

Hello,

When I visit my website I sometimes get the error message: “PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 71 bytes) in …/wp-includes/wp-db.php on line 1539”.

// snipit starting from row 1539
foreach( (array) $this->last_result as $row ) {
	if ( $output == ARRAY_N ) {
	// ...integer-keyed row arrays
	$new_array[] = array_values( get_object_vars( $row ) );
	} else {
	// ...column name-keyed row arrays
	$new_array[] = get_object_vars( $row );
	}
}

When I just refresh the error page it brings me back to my website without any problems. Sometimes the day after I go to it again and the same problem.

I checked my settings and I have the PHP set to 128MB Max, but after checking my usage with a plugin I only use 31% or 39.25MB/128MB.

So I don’t really see where the problem could be. Any help?

Thanks.