Hmmmm....you've done the things that I would have suggested...changed the needle and the tension...another thing I've found is that some threads don't like high speeds. Also, you might try changing your needle again...they do sometimes have flaws...like a rough spot in the eye that we can't detect...but the thread can. I have also been known to unwind a bunch of thread from the current spool...in case I had run into a 'bad' run of thread.....good luck!