Originally Posted by greywuuf

is it failing to catch the bobbin thread when it skips stitches ? if so it means the loop is not being formed big enough for the hook to grab... could be a top tension problem, could be thread hanging in the eye of the needle could be a "catch" or bur somewhere in the thread path..... but the root cause is failure of making the loop.... anything that can cause that is a suspect.

And it sounds like you have checked or changed most of those ... pretty much at a loss needle orientation? wear in the needle bar allowing it to "wander" ?