If you live in the Sacramento Ca area, you need to contact Mizkaki, Cathy, here on the QB. She will not only fix your machine but fine tune it for FMQ.
Now if you don't live in that area or close, then you might consider just straight stitch quilting. You state that the machine works great for piecing. If so, then it should work well for straight stitch quilting. There are many very pretty quilts that are not FMQ, and not all quilts need to be quilted with a FM design.
Also, are you spray basting the quilt? I have found that if I spray baste the quilt then the machine produces skipped stitching. So, I don't do that anymore. I now just hand baste or pin.
Good luck!