The stitch quality is lousy.

Question

The stitch quality is lousy. The bobbin thread is loose and the seam can be pulled apart. I have had it cleaned, adjusted and checked out numerous times by two Bernina authorized dealers ever since I bought it new. I use Mettler or other high quality thread, change the needle more than I changed my children's diapers and nothing helps. Every time I sew I adjust the tension up to 6.5 to be able to sew at all. I'm convinced this machine is defective but no one listens to me.