I generally buy Gutterman for my Viking machines. I do have a bag of my late MILs thread and it is mostly Coats and Clark...some is pretty old, but works in a pinch if I need a particular shade. (I am not using her thread that is on the old wooden spools, however. LOL) I prefer Gutterman..have never had any problems with it. I think you might want to buy a couple small spools of different brands for your DGD just to see if the machine does have a 'preference'. Did you happen to notice what kind of thread the dealer sells? That could definitely be the 'key' to her comment.