background image in textarea moves in IE7 ONLY

Question

Hey,

I am having some trouble with a textarea in IE7. When there is more text than can fit in the height of the textarea, the background image moves upwards. It is 100% in all other browsers (just IE7)

Here is the page; the offending textarea is the message one at the bottom. http://www.nettunes.co.za/build/contact.php

the search bar at the top does a similar thing when you type too many characters into it.

Here is the css for it:

.textarea_input{
    background:url(../images/forms/text_area_back.png) top left no-repeat;
    height:107px;
    width:484px;
    border-style:none;
    font-size:18px;
    color:#cacaca;
    padding-left:10px;  
    padding-top:15px;
}

thanks so much