I actually traded Ina machine that frustrated me (and my dealer who sent it back to the factory, supposedly fixed). I think i, and they, tried all of these things. It was better after the factory check but I simply could not grist it because they could not tell me what they fixed. Lost quite a bit on the trade but am very glad I traded. I told myself it might have been the tension since I do not usually change tension very much for 40 wt poly and prewound bobbins. Since the trade a different dealer told me that shredding usually is on darker thread since it is often dyed-over multiple times. I did not recall if darker thread had been my problem...just glad not to have shredding very often since the trade over a year ago.
Good luck and thank everyone for the excellent comments. I wish I had asked the question when I was having the problem but wasn't using it for quilting.