How about the angle the needle is 'turned' in the shaft? I've had the same problem (and now pretty much refuse to embroider on 'teeshirt' material-just takes too much work) and after checking everything you mention, I find my wife (who does most of the setup) isn't turning a new needle to get the 15 degree or so angle counterclockwise. Also, a tendency to have the plate just a hair to the left of dead center. Once I reset the needle and set the plate back to the right, usually that stops it! Another similar problem was fixed when we started replacing the top rollers at the 2 million stitch mark-worst thread breaks were happening on the rollers with the most wear. Probably a combination of all 3 things but changing it helped!