You have tried most of the things that usually fix this problem. I did have one time that I had obviously knocked the thread tension knob out of it's place. Check were it is currently set and then tighten it all the way in and all the way out a couple of times and then put it back to it's normal setting. I could tell mine was not right because it didn't feel like it was in place properly, there should be a little play but it should be set square in it's place. You might also try running a slightly thicker piece of thread through it to see if there happens to be some lint or a piece of thread stuck. My bobbin case in also a little tricky to get it to seat properly but usually if that is the problem the case will actually fall out while sewing. Good Luck, hope you find the solution.