HTML markup is stored in session

Hello!
I`ve faced a problem, that large amount of HTML markup is stored in sessions table on UPDATE, so it cause a terrific performance issue.
Markup is written at the end of serialized session array. We got memcache installed also. Google says nothing, except some strange topics about invite module slowing down the site.

Bin log is full of rows like this:

/*Some User : sess_write */ UPDATE sessions SET uid = 75715, cache = 0, hostname = 'some_host_name', session = 'image_captcha|a:6:{i:521834116;s:4:\"e1sk\";i:1186741668;s:4:\"6aw2\";i:1084518340;s:4:\"kv25\";i:455724407;s:4:\"jxxj\";i:625258243;s:4:\"mm71\";i:1338914899;s:4:\"9tkp\";}captcha|a:2:{s:13:\"user_register\";a:3:{s:7:\"success\";b:1;s:32:\"cad2b3f80cb09e37920639ed516051eb\";s:4:\"6aw2\";s:32:\"e3ffee5d3f3e73c574e546e9d50f2848\";s:4:\"kv25\";}s:7:\"success\";b:1;}cache_flush|a:2:{s:11:\"cache_block\";i:1254231115;s:10:\"cache_page\";i:1254231115;}og_last|O:8:\"stdClass\":51:{s:3:\"nid\";s:5:\"25878\";s:4:\"type\";s:5:\"panel\";s:8:\"language\";s:2:\"ru\";s:3:\"uid\";s:4:\"3584\";s:6:\"status\";s:1:\"1\";s:7:\"created\";s:10:\"1237287565\";s:7:\"changed\";s:10:\"1241105428\";s:7:\"comment\";s:1:\"0\";s:7:\"promote\";s:1:\"0\";s:8:\"moderate\";s:1:\"0\";s:6:\"sticky\";s:1:\"0\";s:4:\"tnid\";s:1:\"0\";s:9:\"translate\";s:1:\"0\";s:3:\"vid\";s:5:\"40379\";s:12:\"revision_uid\";s:4:\"3341\";s:5:\"title\";s:84:\"From this topic\";s:4:\"body\";s:21791:\"

... LOTS OF MARKUP... looks like whole page is stored ....

\"\";s:3:\"log\";s:0:\"\";s:18:\"revision_timestamp\";s:10:\"1241105428\";s:6:\"format\";s:1:\"1\";s:4:\"name\";s:27:\"Anny Hall\";s:7:\"picture\";s:45:\"sites/default/files/pictures/picture-3584.jpg\";s:4:\"data\";s:1563:\"a:21:{s:7:\"display\";s:1:\"0\";s:10:\"og_reg_key\";s:0:\"\";s:12:\"current_date\";s:10:\"1220528634\";s:10:\"current_id\";s:1:\"2\";s:13:\"form_build_id\";s:37:\"form-d6256db93b610ec83cd03a3107eb7341\";s:16:\"captcha_response\";s:8:\"хтеб\";s:16:\"captcha_solution\";s:8:\"some text\";s:13:\"captcha_token\";s:32:\"4a9a1a19d7f2ddea5d44533ae389e846\";s:12:\"captcha_info\";a:4:{s:7:\"form_id\";s:13:\"user_register\";s:10:\"preprocess\";b:0;s:6:\"module\";s:13:\"image_captcha\";s:4:\"type\";s:5:\"Image\";}s:18:\"birthdays_starsign\";s:6:\"gemini\";s:7:\"contact\";i:1;s:14:\"picture_delete\";i:0;s:14:\"picture_upload\";s:0:\"\";s:34:\"user_relationship_mailer_send_mail\";i:1;s:16:\"guestbook_status\";s:1:\"0\";s:20:\"guestbook_send_email\";i:0;s:15:\"guestbook_intro\";s:0:\"\";s:17:\"messaging_default\";s:4:\"mail\";s:27:\"notifications_send_interval\";s:2:\"-1\";s:18:\"notifications_auto\";i:0;s:5:\"block\";a:12:{s:5:\"block\";a:1:{i:27;i:1;}s:5:\"views\";a:7:{s:17:\"my_groups-block_1\";i:1;s:18:\"groups_new-block_1\";i:1;s:24:\"og_members_block-block_1\";i:1;s:21:\"blog_comments-block_1\";i:1;s:21:\"more_comments-block_1\";i:1;s:21:\"last_comments-block_1\";i:1;s:18:\"og_banners-block_1\";i:1;}s:7:\"archive\";a:1:{i:0;i:1;}s:4:\"user\";a:1:{i:3;i:1;}s:7:\"counter\";a:1:{i:0;i:1;}s:9:\"birthdays\";a:1:{s:12:\"by_birthdays\";i:1;}s:10:\"userpoints\";a:1:{i:0;i:1;}s:8:\"drupalit\";a:3:{i:2;i:1;i:3;i:1;i:4;i:1;}s:10:\"statistics\";a:1:{i:0;i:1;}s:16:\"relevant_content\";a:2:{s:7:\"default\";i:1;s:6:\"groups\";i:1;}s:24:\"user_relationship_blocks\";a:4:{s:4:\"my-1\";i:1;s:4:\"my-2\";i:1;s:16:\"my-3-you_to_them\";i:1;s:16:\"my-3-them_to_you\";i:1;}s:4:\"book\";a:1:{i:0;i:1;}}}\";s:11:\"panels_node\";a:3:{s:3:\"nid\";s:5:\"25878\";s:6:\"css_id\";s:0:\"\";s:3:\"did\";s:5:\"31786\";}s:16:\"field_metod_help\";a:1:{i:0;a:1:{s:5:\"value\";s:1:\"0\";}}s:13:\"field_og_type\";a:1:{i:0;a:1:{s:5:\"value\";s:62:\"some text\";}}s:11:\"field_level\";a:3:{i:0;a:1:{s:5:\"value\";s:52:\"some topic\";}i:1;a:1:{s:5:\"value\";s:50:\"some topic\";}i:2;a:1:{s:5:\"value\";s:63:\"some topic\";}}s:13:\"field_subject\";a:1:{i:0;a:1:{s:5:\"value\";s:23:\"All subjects\";}}s:15:\"field_auditoria\";a:2:{i:0;a:1:{s:5:\"value\";s:16:\"Some role\";}i:1;a:1:{s:5:\"value\";s:43:\"Another role\";}}s:15:\"field_territory\";a:1:{i:0;a:1:{s:5:\"value\";s:97:\"Some group\";}}s:16:\"field_og_archive\";a:1:{i:0;a:1:{s:5:\"value\";s:1:\"0\";}}s:12:\"og_selective\";s:1:\"0\";s:14:\"og_description\";s:276:\"Some add\";s:8:\"og_theme\";N;s:11:\"og_register\";s:1:\"0\";s:12:\"og_directory\";s:1:\"1\";s:12:\"notification\";s:1:\"0\";s:11:\"og_language\";s:0:\"\";s:10:\"og_private\";s:1:\"0\";s:4:\"path\";s:17:\"communities/25878\";s:22:\"last_comment_timestamp\";s:10:\"1237287565\";s:17:\"last_comment_name\";N;s:13:\"comment_count\";s:1:\"0\";s:17:\"epublish_abstract\";N;s:6:\"photos\";a:1:{s:5:\"count\";i:0;}s:8:\"taxonomy\";a:3:{i:34912;O:8:\"stdClass\":5:{s:3:\"tid\";s:5:\"34912\";s:3:\"vid\";s:2:\"16\";s:4:\"name\";s:49:\"some text\";s:11:\"description\";s:0:\"\";s:6:\"weight\";s:1:\"0\";}i:34913;O:8:\"stdClass\":5:{s:3:\"tid\";s:5:\"34913\";s:3:\"vid\";s:2:\"16\";s:4:\"name\";s:20:\"some text\";s:11:\"description\";s:0:\"\";s:6:\"weight\";s:1:\"0\";}i:28545;O:8:\"stdClass\":5:{s:3:\"tid\";s:5:\"28545\";s:3:\"vid\";s:2:\"16\";s:4:\"name\";s:33:\"some text\";s:11:\"description\";s:0:\"\";s:6:\"weight\";s:1:\"0\";}}s:5:\"files\";a:1:{i:16325;O:8:\"stdClass\":13:{s:3:\"fid\";s:5:\"16325\";s:3:\"uid\";s:4:\"3584\";s:8:\"filename\";s:68:\"some text..ppt\";s:8:\"filepath\";s:88:\"sites/default/files/some text..ppt\";s:8:\"filemime\";s:29:\"application/vnd.ms-powerpoint\";s:8:\"filesize\";s:7:\"6012928\";s:6:\"status\";s:1:\"1\";s:9:\"timestamp\";s:10:\"1238425695\";s:3:\"nid\";s:5:\"25878\";s:3:\"vid\";s:5:\"40379\";s:11:\"description\";s:68:\"some text..ppt\";s:4:\"list\";s:1:\"1\";s:6:\"weight\";s:2:\"-1\";}}s:8:\"homepage\";N;s:8:\"realname\";s:46:\"some text\";s:5:\"roles\";a:2:{i:12;s:39:\"some text\";i:14;s:31:\"some text\";}}og_user_role|s:5:\"25878\";', timestamp = 1254231453 WHERE sid = '1541d3as63aa46b3716233136892asd864c85132'

It is also wierd that such amount of parameters are stored.
Any help will be highly appreciated.

Thanks.