Hello Lenny,
First of all if the thread is bunching up it is probably tension problem, go through this systematically and I bet you will get it right, you could have a machine for years and years and get things wrong that one time.
First make sure you have threaded the machine properly and make sure the presser foot is up so the thread goes between the tension discs, make sure you have the needle in the correct way round, tension should be about 4,5 or 6.
Make sure you have the bobbin in the bobbin case the correct way round, it makes a difference, you should just about be able to pull the thread through the bobbin case with some slight tension, if it is too loose you will need to tighten the screw on the bobbin case clockwise about half a turn at a time and test the sewing, try these first.
With the 5137, if you can only sew straight stitch and cannot sew any other stitch, the chances are you have a blunt or bent needle, the needle only needs to be slightly bent and it will miss the stitches especially on zig or any other patterns, if the thread is being pulled out of the needle after a couple of stitches it could be that when you start to sew the takeup lever is in the down position, you must make sure the lever is as the top before you sew, if you are not using a mercerised thread there may be imperfections on the thread that suddenly catch in the eye of the needle to give the impression it is pulling out of the needle, make sure you have the correct size needle for the thread, if you have checked the needle and you are happy that you have threaded the machine properly and the machine misses stitches on zig zag or patterns then the machine could be out of time, but come to that as a last resort.
If you have any problems send me an e-mail.
Ken
drivewithken@blueyonder.co.uk, September 2007