求解，关于纯js写的图片轮流切换问题

鼠标在1234步上来回滑动，图片的切换速度为什么变快了？如何解决此问题，谢了
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>JS+css图片幻灯切换效果丨芯晴网页特效丨CsrCode.Cn</title>
<meta http-equiv="content-type" content="text/html;charset=gb2312">
<!--把下面代码加到<head>与</head>之间-->
<style type="text/css">
*{margin:3;padding:0;font-size:13px;}
body{font-size:12px;color:#333333;text-align:center;font-family:'宋体',arial,verdana,sans-serif;margin-top:8px;}
table{border-collapse:collapse;}
img{border:0;}
a{font-size:16px;}
a:link{color:#333333;text-decoration:none;}
a:visited{color:#333333;text-decoration:none;}
a:hover{color:#FF6600;text-decoration:underline;}
a:active{text-decoration:none;}</style>
<script language="javascript" type="text/javascript">
var _t1 = 0;//打开页面时等待图片载入的时间，单位为秒，可以设置为0
var _t2 = 2;//图片轮转的间隔时间
var _tnum = 4;//焦点图个数
var _tn = 1;//当前焦点
var _tl =null;
var _tt1 ='';
_tt1 = setTimeout ('change_img()',_t1*1000);
function change_img(){
setFocus(_tn);
_tt1 = setTimeout ('change_img()',_t2*1000);
}
function setFocus(i){
if(i>_tnum){
_tn=1;
i=1;
}
_tl?document.getElementById('focusPic'+_tl).style.display='none':'';
document.getElementById('focusPic'+i).style.display='block';
if(_tl)
$("photoNum"+_tl).innerHTML="<a href='#' onMouseOver='focusover("+_tl+");' onMouseOut='focusout("+_tl+");'>第"+_tl+"步</a>";
$("photoNum"+i).innerHTML="<strong onMouseOut='focusout("+i+");'>第"+i+"步</strong>";
_tl=i;
_tn++;
}
function focusover(i){
setFocus(i);
clearInterval(_tt1);
}
function focusout(i){
_tt1=setTimeout ('change_img()',_t2*1000);
}
function $(id){
return document.getElementById(id);
}
</script>
</head>
<body>
<!--把下面代码加到<body>与</body>之间-->
<table width="500" border="2" align="center" cellpadding="0" cellspacing="0">
<tr>
        <td id="photoNum1" width="50px;">
<strong onMouseOut="focusout(1);">第1步</strong>
        </td>
        <td rowspan="4">
<img id="focusPic1" onMouseOver="focusover(1);" onMouseOut="focusout(1);" src="1.jpg" alt="红叶传情">
<img id="focusPic2" onMouseOver="focusover(2);" onMouseOut="focusout(2);" src="2.jpg" alt="红叶传情" style="display:none;">
<img id="focusPic3" onMouseOver="focusover(3);" onMouseOut="focusout(3);" src="3.jpg" alt="红叶传情" style="display:none;">
<img id="focusPic4" onMouseOver="focusover(4);" onMouseOut="focusout(4);" src="4.jpg" alt="红叶传情" style="display:none;">
</td>
</tr>
<tr>
<td id="photoNum2">
<a href="#" onMouseOver="focusover(2);" onMouseOut="focusout(2);">第2步</a>
</td>
</tr>
<tr>
<td id="photoNum3">
<a href="#" onMouseOver="focusover(3);" onMouseOut="focusout(3);">第3步</a>
</td>
</tr>
<tr>
<td id="photoNum4">
<a href="#" onMouseOver="focusover(4);" onMouseOut="focusout(4);">第4步</a>
</td>
</tr>
</table>
</body>
</html>