滚动图片为什么只滚动一轮

Question

 <div class="reply">
            <div class="title">
                <span>title</span></div>
            <div id="demoa" style="float: left; overflow: hidden; width: 980px; height: 300px">
                <div id="demob">
                    <ul>
                        <li>
                            <img src="images/boss/1.jpg" width="115" height="132" border="0" alt="1" /><p>
                                <strong>1<br>
                                    1<br />
                                    1</strong></p>
                        </li>
                        <li>
                            <img src="images/boss/2.jpg" width="115" height="132" border="0" alt="2" /><p>
                                <strong>2<br />
                                    2<br />
                                    2</strong></p>
                        </li>
                        <li>
                            <img src="images/boss/3.jpg" width="115" height="132" border="0" alt="3" /><p>
                                <strong>3<br />
                                    3<br />
                                    3</strong></p>
                        </li>
                        
                    </ul>
                </div>
                <div id="democ">
                </div>
            </div>            <script language="javascript"> 
var speed=30;
democ.innerHTML=demob.innerHTML;
function Marquee(){
    if(democ.offsetTop-demoa.scrollTop<=0){
        demoa.scrollTop-=demob.offsetHeight;
    }
    else{
        demoa.scrollTop++;
    }
}
var MyMar=setInterval(Marquee,speed);
demoa.onmouseover=function() {clearInterval(MyMar)};
demoa.onmouseout=function() {MyMar=setInterval(Marquee,speed)};
            </script>        </div>

Answer 1

没问题啊，我用FF和IE都测试了。滚动不会停止，一直滚动着，从1到3！