I have the 780D and I've noticed it doesn't like certain COLORS of certain brands. I've found two: Like sulky black. or a goldish yellow shade of Madera that is one of my school colors.
If I use the other brand of those two colors, she's fine. Unless I've used the wrong stablilizer. But I can't be mad at her because I don't know what I'm doing...
I've also found a few times as I thread a new color that a "clump" of the last thread color will come out, like it was fraying and about to break. Sounds like the start of your problem, but not quite there yet.