Rubbing alcohol works well for cleaning parts too and is a lot cheaper. Whatever the problem is, it's small. You're very close to having nice looking stitches. Joe's advice is right on. Work on it as the mood strikes you and stop when you hit your frustration point. I'm assuming you have a class 15 bobbin case. You may need to take the spring off it and clean your bobbin case too. A little dirt in there can keep your tension from working properly and give you the results you're seeing. If you have another machine that uses the same case, try swapping cases as a quick test.
Rodney