same query sometimes very slow (punbb forum)

[Gennadiy Barvinok]

Hello,
I have a punbb forum, which uses the following query for displaying topic:

SELECT u.email, u.title, u.url, u.location, u.use_avatar, u.signature, u.email_setting, u.num_posts, u.registered, u.admin_note,
u.reputation_enable, p.id, p.poster AS username, p.poster_id, p.poster_ip, LOWER(p.isocc) as isocc, p.poster_email, p.message,
p.hide_smilies, p.posted, p.edited, p.edited_by, g.g_id, g.g_user_title, o.user_id AS is_online, IsShielded(3,p.poster_id) AS shield,
(select count(*) from reputation rp where rp.post_id = p.id) as repos, (select SUM(r1.rep_plus) from reputation r1
where r1.user_id = p.poster_id) as count_rep_plus, (select SUM(r2.rep_minus) from reputation r2
where r2.user_id = p.poster_id) as count_rep_minus FROM posts AS p INNER JOIN users AS u ON u.id=p.poster_id INNER JOIN
groups AS g ON g.g_id=u.group_id LEFT JOIN online AS o ON (o.user_id=u.id AND o.user_id!=1 AND o.idle=0)
WHERE p.topic_id=14046 ORDER BY p.id LIMIT 50,50;

While this particular query works well and fast (its EXPLAIN is here -- http://www.barvinok.net/auximg/sql_right.png ) all the same query with different topic_id and LIMIT parameters works very slow:

SELECT u.email, u.title, u.url, u.location, u.use_avatar, u.signature, u.email_setting, u.num_posts, u.registered, u.admin_note,
u.reputation_enable, p.id, p.poster AS username, p.poster_id, p.poster_ip, LOWER(p.isocc) as isocc, p.poster_email, p.message,
p.hide_smilies, p.posted, p.edited, p.edited_by, g.g_id, g.g_user_title, o.user_id AS is_online, IsShielded(27,p.poster_id) AS shield,
(select count(*) from reputation rp where rp.post_id = p.id) as repos, (select SUM(r1.rep_plus) from reputation r1
where r1.user_id = p.poster_id) as count_rep_plus, (select SUM(r2.rep_minus) from reputation r2
where r2.user_id = p.poster_id) as count_rep_minus FROM posts AS p INNER JOIN users AS u ON u.id=p.poster_id INNER JOIN
groups AS g ON g.g_id=u.group_id LEFT JOIN online AS o ON (o.user_id=u.id AND o.user_id!=1 AND o.idle=0)
WHERE p.topic_id=8993 ORDER BY p.id LIMIT 25050,50;

and in this case, EXPLAIN looks like that: http://www.barvinok.net/auximg/sql_wrong.png

It seems that the problem lies within 'users' table, which is defined as

CREATE TABLE `users` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `group_id` int(10) unsigned NOT NULL DEFAULT '4',
  `username` varchar(200) NOT NULL DEFAULT '',
  `password` varchar(40) NOT NULL DEFAULT '',
  `email` varchar(50) NOT NULL DEFAULT '',
  `title` varchar(50) DEFAULT NULL,
  `realname` varchar(40) DEFAULT NULL,
  `url` varchar(100) DEFAULT NULL,
  `jabber` varchar(75) DEFAULT NULL,
  `icq` varchar(12) DEFAULT NULL,
  `msn` varchar(50) DEFAULT NULL,
  `aim` varchar(30) DEFAULT NULL,
  `yahoo` varchar(30) DEFAULT NULL,
  `location` varchar(30) DEFAULT NULL,
  `use_avatar` tinyint(1) NOT NULL DEFAULT '0',
  `signature` text,
  `disp_topics` tinyint(3) unsigned DEFAULT NULL,
  `disp_posts` tinyint(3) unsigned DEFAULT NULL,
  `email_setting` tinyint(1) NOT NULL DEFAULT '1',
  `save_pass` tinyint(1) NOT NULL DEFAULT '1',
  `notify_with_post` tinyint(1) NOT NULL DEFAULT '0',
  `show_smilies` tinyint(1) NOT NULL DEFAULT '1',
  `show_img` tinyint(1) NOT NULL DEFAULT '1',
  `show_img_sig` tinyint(1) NOT NULL DEFAULT '1',
  `show_avatars` tinyint(1) NOT NULL DEFAULT '1',
  `show_sig` tinyint(1) NOT NULL DEFAULT '1',
  `timezone` float NOT NULL DEFAULT '0',
  `language` varchar(25) NOT NULL DEFAULT 'English',
  `style` varchar(25) NOT NULL DEFAULT 'Oxygen',
  `num_posts` int(10) unsigned NOT NULL DEFAULT '0',
  `last_post` int(10) unsigned DEFAULT NULL,
  `registered` int(10) unsigned NOT NULL DEFAULT '0',
  `registration_ip` varchar(15) NOT NULL DEFAULT '0.0.0.0',
  `last_visit` int(10) unsigned NOT NULL DEFAULT '0',
  `admin_note` varchar(30) DEFAULT NULL,
  `activate_string` varchar(50) DEFAULT NULL,
  `activate_key` varchar(8) DEFAULT NULL,
  `reputation_enable` smallint(6) DEFAULT '1',
  `reputation_enable_adm` tinyint(1) unsigned DEFAULT '1',
  `clone` int(11) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `users_registered_idx` (`registered`),
  KEY `users_username_idx` (`username`(8)),
  KEY `statidx` (`last_post`,`username`,`id`)
) ENGINE=MyISAM AUTO_INCREMENT=8874 DEFAULT CHARSET=cp1251 ROW_FORMAT=FIXED

Any suggestions how to optimize the query?