Ok I am going to jump in here with another idea. Read this thread here on the board.
http://www.quiltingboard.com/main-f1...ng-t35165.html
Could it be tufts of batting showing through on the back of your quilt and not the tension. Also my sewing machine man years ago showed me the drop test with the bobbin but he showed not to have it drop over a quarter inch at a time. I hope this helps you some.