It could be your bobbin itself. I wound some bobbins the other day for my Viking and one of them just did not come out right. Sure enough, I had birds nests and breaking thread. As soon as I changed bobbins everything was a-ok.
I have an Ambition and really love it. The drop-in bobbin is identical to my Viking. Make sure your bobbin thread "clicks" through the first part and ishould be correct. Stay with your Ambition until this one problem is solved and I think you will love it to.
Oh the only finicky part of this machine - make sure you buy a needle plate with the single hole for your straight sewing and piecing. It wants to either chew the fabric or nest it at the beginning when using the zig zag plate.