You could try using a seam jumper when crossing bulk, this is a little plastic device that you put in front and behind the pressure foot to keep it level as it goes over the bulk. This then means the machine can keep feeding the fabric evenly. WIthout it, the feed dogs arent getting enough pressure, the machine slows and can't feed the seam, your needle may deflect and strike the foot, and broken needle may be the result.
Here is a link to one called a Jean a Ma Jig, http://sewing.about.com/gi/dynamic/offsite.htm?site=http://www.clotilde.com/store/
Also, try bashing the seam you are wanting to sew across first with a hammer and always trim the seam allowances inside the seam if you can. If sewing denim, use a denim needle with blue coating, it helps it pass between the densely woven fibres in this fabric.
However, it may be that your machine just hasn't got the strength to stitch through more than 6 layers, many wont. Especially if you use buttonhole twist thread to get the read to wear jeans seam finish. I often resort to using my old SInger 20U industrial straight stitch when altering jeans.