TAG in article are truncated

here is the line i'm inserting in an article:

<div style="text-align: center">
<a href="reservation1.php?KeepThis=true&amp;TB_iframe=true&amp;height=500&amp;width=700&amp;modal=true" class="thickbox" title="FORMULAIRE DE RESERVATION"><img src="images/Boutons/reserver.gif" alt="" /></a>
</div>

this line invokes the creation of a button at the end of the article. This button should execute the opening of a thickbox and should display a basic html form.
**BUT**
those attributes ": class="thickbox" title="FORMULAIRE DE RESERVATION" disapear each time i save (following changes) the article

to resume:
i'm saving (it is saved as this in th database the first time):
<div style="text-align: center">
<a href="reservation1.php?KeepThis=true&amp;TB_iframe=true&amp;height=500&amp;width=700&amp;modal=true" class="thickbox" title="FORMULAIRE DE RESERVATION"><img src="images/Boutons/reserver.gif" alt="" /></a>
</div>

and when i come back to the article, it is displayed like that:
<div style="text-align: center">
<a href="reservation1.php?KeepThis=true&amp;TB_iframe=true&amp;height=500&amp;width=700&amp;modal=true" ><img src="images/Boutons/reserver.gif" alt="" /></a>
</div>

any help?????

regards