Years ago I made what I called The Christmas Wallhanging from Hell. It was really cute until I went to quilt it with metallic thread. No problems for the first half, but after awhile the thread broke over and over again. Nothing I did helped. I finally gave up in frustration and it turned into a UFO. Periodically I would pull it out and try again, but no go -- the metallic thread broke constantly. Many years later I ran across it and gave it another try -- same thread, same machine, same wallhanging. No problems.
My best guess is that there was a tiny bit of metallic lint stuck somewhere in the bobbin area or upper tension discs of the machine. It was not enough to interfere with other threads, but the metallic thread went ballistic with it. Over time and with lots of other sewing, the lint finally dislodged.
Is it too late to switch to a different metallic thread? I have heard that Glide's metallic thread is much less likely to break (although judging by the following link, this is not always true). Here is a link that may be helpful to you:
http://rhondabracey.com/2014/04/29/h...allic-threads/
Note: She mentions at the bottom of that website that it is very important to use a thread net. The thread net adds tension, so reduce upper tension a little to compensate. Plus Sewers Aid seems to help.