If the 500 has been sewing well, and this is a new problem, I would look elsewhere for the answer. Is the thread old? Pull the spool and do a complete rethreading. Check the upper tension setting, and look to see if the thread is between the right discs. Is the needle inserted with the flat side facing the correct direction? Is it the same needle as before, or is this a new needle? Sometimes needles (eyes) are missed in finishing.
There may be some other possible causes, and others will chime in. Let us know what you discover, please.