Try reducing your needle tension. Because monofilament thread has stretch is creates its own "tension" (think about if you were sewing with the old elastic thread) as it goes through your machine. By reducing the needle tension (I always reduce my by 2 whole numbers on my Viking D1 and then fine tune from there).
If you use monofilament in your bobbin, remember to wind slowly. If your bobbin is plastic wind somewhere around half full. I've had a plastic bobbin pop open because I wound too much thread on it. If the monofilament gets warm while winding fast...it can stretch as its wound. That will increase the probability of the bobbin thread breaking, too.
The alternative for the bobbin is to change to a nice 50 wt cotton like Aurifil or Masterpiece. They will look nice and play nicely with the monofilament.
Hope this helps...
Julie