Sorry you are having problems. I know how frustrating it can be.
I read your post as this is a new predicament for the machine so are the fabrics different from what you have quilted before? Regular cotton fabrics are more forgiving than Batiks. Thread Different? You might try using the next size up needle or use a top stitch needle which has a longer eye and is a bit more sturdy and forgiving as that may help.
Other things that could cause you trouble are burs in the tension assembly, a bent needle or as Tartan commented the needle hitting the stitch plate (but if that were the case - I would think that the repair man would have caught that) . Older thread might also cause problems as well. If it were me, I would try a new needle, a new spool of thread and a new bobbin freshly filled and see if that gets you anywhere. I might even try a practice quilt sandwich with different fabrics.