Courtney, I have the evil twin of your machine
It is currently at the store where I purchased it from for the third time. I saw your problems and they exactly match what I experience with my machine, the thread breaking, problems going side to side. I also have had problems with skipped stitches. These problems mainly occurred with Superior SoFine 40wt thread. I would happily quilt dense background stippling with bottom line in the top and bottom and when I changed to the heavier weight thread for a different part of the quilt, everything went downhill. Luckily I have another machine (Janome 6600P) it worked beautifully with all the threads and I was able to finish the quilts in time for Christmas. I tried everything that I could think of - lots of different needles, tension changes in the top and bobbin, different weight thread in the bobbin... The repair man told me to take my threads in and he would test them all. I pick the machine up tomorrow and will meet with the repair man when I collect it so he can go over everything with me. I will let you know what feedback I get. Hopefully it will help you too.