The Freezed header disapper when scrolldown the gridview and then try to change label

[bipiney]

Hi,
In my aspx page I have a gridview inside a panel as below

<asp:Panel runat="server" ID="pnlContainer" ScrollBars="Auto" Height="781px" Width="1250px">
        <asp:GridView ID="gvCashFlowDtls" AutoGenerateColumns="False" runat="server" Font-Size="Small"
            DataKeyNames="UID" Width="98%" 
            onrowdatabound="gvCashFlowDtls_RowDataBound">
            <HeaderStyle CssClass="GVFixedHeader" />
            <Columns>

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  </Columns>            
        </asp:GridView>
    </asp:Panel>

This is the style I used to freeze the grid view header.
<style type="text/css">
        .GVFixedHeader
        {
            font-weight: bold;
            background-color: #BFBFBF;
            cursor: default;
            position: relative;
            top: BLOCKED EXPRESSION;
            z-index: 10;
        }
</style>

The header is freezed as I expected. In the gridview I have a label in one of the template columns.I have specified a cssclass for the label,the class has no definition in my style.The class is just for getting the array of the labels in jquery.I retrieve the array of the lable like this
var allocbud = $(".allocbud"); in my jquery.Based on some calculations I set the value of the label in a jquery function.It is working fine. The problem occurs when I scroll down the gridview with freezed header and then try to change the value of the label.The Freezed header disappers!!! Then if I scroll back to the normal position and then change the value, the header reappears!!

Please help me to fix this issue.

[bipiney]

The top expression was not complete in the previous post
<style type="text/css">
.GVFixedHeader
{
font-weight: bold;
background-color: #BFBFBF;
cursor: default;
position: relative;
top: expression(parentNode.parentNode.parentNode.parentNode.parentNode.scrollTop-2);
z-index: 10;
}
</style>