Go reset your stitch settings if you have saved them in personal stitches along with trying everything else. I have some stitches saved in personal stitches on the 2170 and they will stitch funny at times. Delete, reprogram and save again.
You just might have a weird machine. I had an Elna serger do that. I would have it set up just right, sewing fine, turn the machine off. Come back next day to use the same settings, and the stitches were totally haywire. Threading, needles, settings just fine when turned off. Turn on and no go.