You only need to insert the embedding code into your page once, and there is no need for adding any iframe stuff into your html code. So, in the place where you currently put the nabble code, simplify it to:
<a id="nabblelink" href="http://n2.nabble.com/vandweller%27s-SNG-Forum-f1344067.html" style="display: none;">vandweller's SNG Forum</a>
<script src="http://n2.nabble.com/embed/f1344067"></script>
That should fix the problem.