Try switching to a 90/14 or even a 100/16 Topstitch needle.
also, if you are using the free motion foot that floats instead of hopping, it might help to use the thumb screw on the foot to lower it a little. That foot needs to skim the surface, or float just a tiny bit above it. If it's too far off the surface you will get problems with stitch formation and thread breakage.
Rob