Because we restore vintage machines we get a lot of questions like this. Quality and type of needle used has a profound impact. Go to Schmetzneedles.com for a guide.
Tension set too high can also put tork on the thread and needle especially in free motion. Many times free motion can be done with a presser foot attached and that helps stablized the pull and drag of the thread through the fabric from side to side etc.
Another good thing to look up is sewing Machine trouble shooting on the web- there are many places that give you a comprehensive list. If you're able to do a straight stitch with the machine, more likely then not it is something to do with your motion and or tension. You really don't realize how much pressure is being exerted against the needle when you free motion.