请教问题。。。

<div id="rab" style="margin-top: 10px"> 
                            <div id="rab1" style="margin-top: 0px"> 
                                图片 </div> 
                            <div id="demo" style="overflow: hidden; width: 99%; height: 100%" align="center"> 
                                <table cellpadding="0" align="left" width="100%" border="0" cellspace="0"> 
                                    <tr> 
                                        <asp:Repeater ID="ImgnewsList" runat="server"> 
                                            <ItemTemplate> 
                                                <td id="demo1" valign="middle"> 
                                                    <a href='xxxw.aspx?oid= <%# DataBinder.Eval(Container.DataItem, "OID").ToString()%>' 
                                                        target="_self"> 
                                                        <img src=" <%# "admin/"+DataBinder.Eval(Container, "DataItem.address") %>" alt=" <%#DataBinder.Eval(Container, "DataItem.Title") %>" 
                                                            style="width: 135px; height: 120px" /> 
                                                    </a> 
                                                </td> 
                                                <td id="demo2" valign="middle"> 
                                                </td> 
                                            </ItemTemplate> 
                                        </asp:Repeater> 
                                    </tr> 
                                </table> 
                            </div> 
                            <div>                                 <script type="text/javascript"> 
                            var Picspeed=50 
                            demo2.innerHTML=demo1.innerHTML 
                            function Marquee1(){ 
                            if(demo2.offsetWidth-demo.scrollLeft <=0) 
                            demo.scrollLeft-=demo1.offsetWidth 
                            else{ 
                            demo.scrollLeft++ 
                            } 
                            } 
                            var MyMar1=setInterval(Marquee1,Picspeed) 
                            demo.onmouseover=function() {clearInterval(MyMar1)} 
                            demo.onmouseout=function() {MyMar1=setInterval(Marquee1,Picspeed)} 
                                </script>                             </div> 
                        </div> 请问为什么首尾相撞滚动两次之后就会停止是什么原因？