I have been having some trouble with my machine lately. It is skipping stitches and bunching in random places. Sometimes it helps when I stop and re-thread my machine, but I am wondering if I am just doing something silly or wrong. What do you ladies know about machine needles? Should I be more diligent about changing them? Also, is there a rule of thumb (or thimble?) about when to use what kind of needle? How about thread? Are there certain projects when I should use synthetic (polyester) or cotton? Does any of this really make a difference, or might there be something wrong with my machine?