There was a series of backs for the Super that had some issues like you describe... Basically they would were out too soon and start having winding issues. I used to remember how to tell if it was an early or late model back, but not anymore.

Also the 120/220 is really all in the insert, so you should be able to put the 220 insert into the suspect back and see if the problem is still there. If the problem is gone than you just need a 120 insert. You could try the same with the 120 insert in the second back too to check the insert.