Post save timeout

Hi,

When clicking save/post button in post editor the page loads for a few minutes and finally gives an error:

WordPress database error: [Lost connection to MySQL server during query]
select tag from m_tags where tag = ‘sometag’
Warning: Cannot modify header information – headers already sent by (output started at /usr/local/apache/www/htdocs/service.miejsca.org/wp-includes/wp-db.php:104) in /usr/local/apache/www/htdocs/service.miejsca.org/wp-includes/pluggable-functions.php on line 270

The post is saved nevertheless, but it is very annoying. I tried to turn all plugins off, repair database and clear tags, with no effect.

Would you please help? I can live with it, but most editors simply can’t.