I have a Juki that I use on a Grace frame. I am completely self taught. I had a lot of problems when I first started. I had problems with top thread breaking and bottom thread tangling. It seemed like it took me a long time, but I finally have everything working well. Here is what I do:
I use sewer's aid on the top thread
I use a size 14 needle
I use bobbin genies in the bobbin case
You could also be having a problem with your tension. There should be a section in your manual that shows examples of what the stitching should look like on the top and bottom. You can use that to try to determine if you need to adjust the top tension or the bobbin tension. You may need to adjust both. If you have already followed the directions on making sure the bobbin case is adjusted correctly, I would start by making a small adjustment on the case. Then I would do a test stitch. You may have to do this several times before you see a difference. You can do the same with the top thread. This may take some time and patience, but pretty soon you should see enough of a difference in the stitching that you will get a feel for what needs to be done with the tension.
Feel free to pm me if this doesn't make sense.
I'm sure there are others with more experience with the Juki that can give you more ideas. I'm going to check back to see what other suggestions you get - I'm sure I have a lot to learn
Good luck and let us know what finally works.