what size needle are you using? I would suggest a size UP. If I think I will have a heavier quilt sandwich, I use an 18. I try to do a test piece and get my tensions to my liking. Jamie Wallen has a wonderful youtube video on machine quilting tensions.
http://www.youtube.com/watch?v=Q1mRhcquZTM This has saved me more than once!
I just had a shredding issue...turns out I had my top tensions too tight. PM if you want...