Sloooow site, database errors

Hi. The performance of my (Drupal 5.7) site has been degrading a lot in the last few days.

Given that the only changes have been core and module upgrades, and the reader traffic has actually decreased, from more than 150.000 unique users a month down to around 130.000 users, I guess there's some problem with the code itself.

Another symptom is that the number of connected users (according to Drupal) keeps growing to previously unknown levels, while the site gets slower and slower until the DB finally times out and Drupal displays the dreaded 'Unable to connect to database' screen with a warning about 'too many connections'. Most content posting operations get broken uncompleted, with user sessions disconnected.

Of course, Drupal caching is enabled, as is throttle for most of the modules.

My hosting support have said:

currently your server is executing 210+ mysql queries per second. I just checked one shared server hosting 500+ sites and found it getting ~20 mysql queries per second only

and they have given up.

I have noticed several PHP, DB-related entries in the watchdog, but I'm unable to identify their source:

Lost connection to MySQL server during query query: SELECT sid FROM sessions WHERE sid = '5bfb6670512c7ad8a2f11ebf7389b4ef' in line 173 of file /home/MYSITE/public_html/includes/database.mysql.inc.

Line 173 says:

trigger_error(check_plain(mysql_error($active_db) ."\nquery: ". $query), E_USER_WARNING);
    return FALSE;

Another error:

mysql_fetch_object(): supplied argument is not a valid MySQL result resource in /home/MYSITE/public_html/includes/database.mysql.inc on line 189.

Line 189 says:

return mysql_fetch_object($result);

Another error:

Table 'temp_search_sids' already exists query: CREATE TEMPORARY TABLE temp_search_sids SELECT i.type, i.sid, SUM(i.score * t.count) AS relevance, COUNT(*) AS matches FROM search_index i INNER JOIN search_total t ON i.word = t.word INNER JOIN node n ON n.nid = i.sid LEFT JOIN i18n_node i18n ON n.nid = i18n.nid INNER JOIN users u ON n.uid = u.uid WHERE n.status = 1 AND (i18n.language ='es' OR i18n.language ='' OR i18n.language IS NULL) AND (i.word = 'ipaq') AND i.type = 'node' GROUP BY i.type, i.sid HAVING COUNT(*) >= 1 en la línea 173 del archivo /home/MYSITE/public_html/includes/database.mysql.inc.

Yet another one:

mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/MYSITE/public_html/includes/database.mysql.inc on line 219.

Line 219 says:

return mysql_num_rows($result);

And so on. Can somebody here help in cleaning things up? Any assistance will be much appreciated.

Thanks in advance
--
Albert