DropDownList losing CSS

Hi,

I'm having this problem: the dropdownlist is losing CssClass on Postback inside a UpdatePanel.

My CssClass : 

.selectBox-dropdown { color: #8e8a8c; font-size: 13px; font-weight: bold; font-family: 'Droid Sans', Arial, Helvetica, sans-serif; min-width: 150px; position: relative; /*line-height: 2;*/ height: 16px; padding: 5px 0; text-decoration: none; text-align: left; outline: none; vertical-align: middle; background: #f6f6f6; -moz-box-shadow: inset 1px 1px 2px #efefef; -webkit-box-shadow: inset 1px 1px 2px #efefef; box-shadow: inset 1px 1px 2px #efefef; -webkit-border-radius: 4px; -moz-border-radius: 4px; border-radius: 4px; display: inline-block; cursor: default; }

.selectBox-dropdown:focus,

.selectBox-dropdown:focus .selectBox-arrow { border-color: #666; }

.selectBox-dropdown .selectBox-menuShowing { -moz-border-radius-bottomleft: 0; -moz-border-radius-bottomright: 0; -webkit-border-bottom-left-radius: 0; -webkit-border-bottom-right-radius: 0; border-bottom-left-radius: 0; border-bottom-right-radius: 0; }

.selectBox-dropdown .selectBox-label { padding: 2px 8px; display: inline-block; white-space: nowrap; overflow: hidden; }

.selectBox-dropdown .selectBox-arrow { position: absolute; top: 0; right: 0; width: 43px; height: 100%; background: url(../images/others/arrow-select.jpg) 50% center no-repeat; }

My dropDownList :

<asp:DropDownList ID="cboBuscarMaquina" CssClass="selectBox-dropdown" runat="server"><asp:ListItem Text="Nr. de Série" Value="MAQNUMBER"></asp:ListItem><asp:ListItem Text="Modelo" Value="MAQTYPE"></asp:ListItem><asp:ListItem Text="Cliente" Value="CLIENTEUNIDADE.CLIUNNOME"></asp:ListItem><asp:ListItem Text="Stations / Type Punches" Value="MAQSTATIONS"></asp:ListItem><asp:ListItem Text="Die / Segment" Value="MAQDIEORSEGMENT"></asp:ListItem></asp:DropDownList>

And after a postback I loss all CssClass except the first.

Anyone can help me??

Thanks...