Empty post data, multiple instances

Hi,
I've been using Ckeditor for a few years and it's the first time that I found a problem that I couldn't figure out by myself.

On my page I have a multiple instances of ckeditor and last week I started to have a problem: when I submit my form, a post data from Ckeditor textarea's are empty. Sometimes it's work fine (script receive the data), but usually it doesn't. I didn't change anything in my scripts (especially in ckeditor files). I updated ckeditor to the newest (CKEditor 3.6.1 revision 7072) version but it makes no difference. Here's my code:

Textareas:
<dt id="text_en-label"><label for="text_en" class="required">English:</label></dt>
<dd id="text_en-element"><textarea name="text_en" id="text_en" rows="24" cols="80"></textarea></dd>
<dt id="text_pl-label"><label for="text_pl" class="required">Polish:</label></dt>
<dd id="text_pl-element"><textarea name="text_pl" id="text_pl" rows="24" cols="80"></textarea></dd>

Ckeditor implementation:
<script type="text/javascript">CKEDITOR.replace( 'text_en' );</script>
<script type="text/javascript">CKEDITOR.replace( 'text_pl' );</script>

Ckeditor config:
CKEDITOR.editorConfig = function( config )
{
config.language = 'pl';
config.skin = 'office2003';

config.filebrowserBrowseUrl = '/js/ckfinder/ckfinder.html';
config.filebrowserImageBrowseUrl = '/js/ckfinder/ckfinder.html?Type=Images';
config.filebrowserFlashBrowseUrl = '/js/ckfinder/ckfinder.html?Type=Flash';
config.filebrowserUploadUrl = '/js/ckfinder/core/connector/php/connector.php?command=QuickUpload&type=Files';
config.filebrowserImageUploadUrl = '/js/ckfinder/core/connector/php/connector.php?command=QuickUpload&type=Images';
config.filebrowserFlashUploadUrl = '/js/ckfinder/core/connector/php/connector.php?command=QuickUpload&type=Flash';

config.toolbar_Full =
[
['Source','-','Save','NewPage','Preview','-','Templates'],
['Cut','Copy','Paste','PasteText','PasteFromWord','-','Print', 'SpellChecker', 'Scayt'],
['Undo','Redo','-','Find','Replace','-','SelectAll','RemoveFormat'],
['Form', 'Checkbox', 'Radio', 'TextField', 'Textarea', 'Select', 'Button', 'ImageButton', 'HiddenField'],
'/',
['Bold','Italic','Underline','Strike','-','Subscript','Superscript'],
['NumberedList','BulletedList','-','Outdent','Indent','Blockquote','CreateDiv'],
['JustifyLeft','JustifyCenter','JustifyRight','JustifyBlock'],
['Link','Unlink','Anchor'],
['Image','Flash','Table','HorizontalRule','Smiley','SpecialChar','PageBreak'],
'/',
['Styles','Format','Font','FontSize'],
['TextColor','BGColor'],
['Maximize', 'ShowBlocks','-','About']
];

config.toolbar_Basic =
[
['Preview'],
['Cut','Copy','Paste','PasteText','PasteFromWord'],
['Undo','Redo','-','Find','Replace','-','SelectAll','RemoveFormat'],
['JustifyLeft','JustifyCenter','JustifyRight'],
['Image','Table','HorizontalRule','Smiley','SpecialChar','PageBreak'],
'/',
['Bold','Italic','Underline','Strike','-','Subscript','Superscript'],
['NumberedList','BulletedList','-','Outdent','Indent','Blockquote'],
['TextColor','BGColor'],
['Link','Unlink','Anchor'],
'/',
['Styles','Format','Font','FontSize'],
['Maximize', 'ShowBlocks']
];
};

I also use a button outside my form to submit the data:
<a href="#" onClick="document.getElementById('admin_form').submit(); return false;" class="button"><img alt="Submit Form" src="/admin_template/images/icons/confirm.png" /><br />Submit</a>

I will appreciate any help,
Best Regards