I have only had trouble with one kind of thread. Metler. I think it's more the thread than the Juki. If I set the thread on the floor and have it come through a thread stand, then to my juki, then it's ok. I won't buy that brand any more once my current spools are gone. Although I do like the finished results with the metler...
Good Luck with your Juki. Oh, yes, on my blog sidebar, I save the blogs of other Juki TL98q girls. They are very nice and it is through one of these Juki girls, that I found a source for my quarter inch feet. I'm going to order them in two weeks when I go south for the winter.