sumando automaticamente

[sitges65]

La verdad es que sigo sin saber como variarlo.....
<script type="text/javascript">
function Sumar(){
interval = setInterval("calcular()",1);
}
function calcular(){
tres = document.autoSumForm.Select2.value;
dos = document.autoSumForm.Select1.value;
document.autoSumForm.TerceraCaja.value = (tres * 1) + (dos * 1);
}
</script>
<form name="autoSumForm">
<select name="Select2">
<option selected="" value="0" onfocus="Sumar">NINGUNO</option>
<option value="20" onfocus="Sumar();" onBlur="NoSumar();">B</option>
<option value="30" onfocus="Sumar();" onBlur="NoSumar();">C</option>
<option value="40" onfocus="Sumar();" onBlur="NoSumar();">D</option>
<option value="10" onfocus="Sumar();" onBlur="NoSumar();">A</option>
</select><br>
<select name="Select1">
<option selected="" value="0"onfocus="Sumar();" onBlur="NoSumar();">none</option>
<option value="400"onfocus="Sumar();" onBlur="NoSumar();">q</option>
<option value="800"onfocus="Sumar();" onBlur="NoSumar();">t</option>
</select><br>
<br>
<input type="text" onfocus="Sumar()" value="" name="TerceraCaja">
</form>

ya debo estar cerca porque me funciona solamente si hago click en el cuadrito total..a partir de ahí los calculos ok

help!!