As a wild guess, something is catching the thread on the way down. If you have changed needles, threads and cleaned machine, this is the only place it could be weakening the thread. It may be that something has tweaked or moved just a tad and is catching. If you can't see inside or take the cover off, it would be worth it to take it into a shop and asking them. I know that one time I was having a devil of a time with assorted problems and discovered a 1/4" piece of thread caught inside the bobbin area! Just about drove me nuts!!!