Estimado MarkSock, realizé lo que has sugerido y el problema se mantiene, te mando el codigo completo haber si aparace algo, el ejemplo es de solo dos multiplicaciones.
<html>
<head>
<title>Ensayo</title>
<SCRIPT>
function calculate()
{
var val1,val2,valsub,sumar,restar,res1,res2;
val1 = ((document.calcform.price1.value)*(document.calcfo rm.quantity1.value))
val2 = ((document.calcform.price2.value)*(document.calcfo rm.quantity2.value))
valsub = ((val1) + (val2))
document.calcform.subtotal.value = valsub
sumar = parseInt(document.calcform.sumarle.value);
res1= ((valsub) + (sumar))
document.calcform.total.value = res1
restar = parseInt(document.calcform.descuento.value);
res2= ((valsub) - (restar))
document.calcform.total.value = res2
}
</SCRIPT>
</head>
<body onload="this.document.calcform.price1.focus()","ca lculate">
<FORM name="calcform">
<table width="80%" border="0" align="center">
<tr>
<td><TABLE width="50%" border=0 align="center" class="Yuyo" >
<TR>
<TD width="32%"><center>Valor </center></TD>
<TD width="32%"><center>Cantidad</center></TD>
</TR>
<TR>
<TD><INPUT type="text" name="price1" ONCHANGE="calculate()"></TD>
<TD><INPUT type="text" name="quantity1" ONCHANGE="calculate()"></TD>
</TR>
<TR>
<TD><INPUT type="text" name="price2" ONCHANGE="calculate()"></TD>
<TD><INPUT type="text" name="quantity2" ONCHANGE="calculate()"></TD>
</TR>
</TABLE>
<p> </p>
<table width="46%" border="0" align="center" class="Yuyo">
<tr>
<td><div align="right"></div></td>
<td colspan="2"><center>
</center></td>
</tr>
<tr>
<td><div align="right">Subtotal</div></td>
<td colspan="2"><center>
<INPUT type="text" name="subtotal" >
</center></td>
</tr>
<tr>
<td><div align="right">Sumarle:</div></td>
<td colspan="2"><center>
<input type="text" name="sumarle">
</center></td>
</tr>
<tr>
<td><div align="right">Restarle:</div></td>
<td colspan="2"><center>
<input type="text" name="descuento" >
</center></td>
</tr>
<tr>
<td><div align="right">Total:</div></td>
<td colspan="2"><center>
<INPUT type="text" name="total">
</center></td>
</tr>
<tr>
<td colspan="3"><center>
</center></td>
</tr>
<tr>
<td colspan="3"><center>
<INPUT name="button" type="button" ONCLICK="calculate()" Value="Calcular">
</center></td>
</tr>
<tr>
<td colspan="3"><div align="right"></div>
<center>
</center></td>
</tr>
</table>
<p> </p></td>
</tr>
</table>
</form>
</body>
</html>