I got myself one of those tall thread holder that pulls the thread vertical and I slow down the speed some. I use to have horrible time with metallic too, but since them it is working great. Also make sure you use a metallic needle too. I have found out that Marathon Threads doesn't shred like others and they have a Fabulouso irredescent thread that is out of this world. I use it on a lot of the santas.
Hope this helps.
R