pp_1 on "Wordpress Plugins and custom mySQL query conflict"

Hi,

What's wrong: When I use mySQL custom query on one of pages AND I have some plugins I experience a conflict.

My custom query works well, but plugins show errors (Can't connect to gallery, can't find gallery etc.) or don't show up at all.
Custom query OR plugin -> everything's just fine.

An example of code:

$username= "wordpress_user";
$password = "wordpress";
$hostname= "localhost";

$dbhandle1 = mysql_connect($hostname, $username, $password)
or die("Unable to connect to MySQL");

$selected = mysql_select_db("denco_1",$dbhandle1)
or die("Could not select examples");

$result = mysql_query("SELECT szkolenia.temat, wykladowcy.nazwa, wykladowcy.oznaczenie, wykladowcy.id AS wykladowca_id, szkolenia.specjalnosc, szkolenia.id, szkolenia.pokaz FROM szkolenia LEFT JOIN wykladowcy ON szkolenia.prowadzenie = wykladowcy.oznaczenie WHERE szkolenia.pokaz = 'tak'");

if(mysql_num_rows($result) > 0) {
while($row = mysql_fetch_object($result)) {
echo "$row->specjalnosc"; }

Plugin code examples:

[nggallery id=1]

or

<?php do_action('slideshow_deploy', '74'); ?>

Doesn't matter if plugin (or widget) is in the same column with query or in the sidebar / footer. It seems that plugins that need an sql connection don't work when I use it...

What I've already tried:
-Wordpress update (twice, two last versions) -no difference;
-My theme (Atahualpa) update -no difference;
-Switch theme to Twenty Ten and Eleven - no difference;
-rename $username, $password, $hostname and $dbhandle variables -no difference;
-change php handling plugin (3 different)- no difference;
-import tables I need to wordpress database -no difference;
-import tables I need to wordpress database AND change
$result = mysql_query("Select...");
to
$result = $wpdb->get _results("Select...)-no difference
-add and remove
mysql_close(); -no difference;

I'm out of ideas, I'd appreciate your help very much.