EVERY TIME I CHANGE THE BOBBIN!!! lint builds up quickly and can cause all kinds of problems, when i purchased my machine i was told how important it is to keep it clean...I paid alot of money for it and want it to last for many years; so;
every single time i change the bobbin i take it apart and clean it with a brush, then between each project (sometimes twice if it's a big project) i vacuum it all out as best i can. then the shop kind of got after me for not taking it in for a full servicing at least every 6 months...i still don't manage that, it only makes it once a year...since it's 90 miles one way to take it in...but i take pretty good care of it myself between servicing,.