I had the same thing happen with my machine a few weeks ago just out of the blue. Made me crazy!!! As it turned out...I tried all the things mentioned above EXCEPT I didn't take the bobbin case out.........WELLLLLLLLL when I finally did that there was a piece of FUZZ hidden... After I cleaned the bobbin area out really good.....PRESTO! Back to stitch'en properly. Just keep playing...... IT WILL go back to normal..... Quilters are tested every day. :) :) :)
Good luck!