You want something like
<xsl:for-each select="Task[@due='true']">
....
<xsl:if test="position() mod 5 = 1">
<xsl:text>---------------</xsl:text>
</xsl:if>
</xsl:for-each>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference