Hi
It's hard to tell without having hands on the machine. This could be a problem with the thread rather than the machine. If you happen to be using a large spool of cross wound thread this could make the thread feed erratic putting loops in the thread which will tangle. Have you had this machine sewing well in the past? You could do a test with another thread. If you want to use large spools of cross wound thread on a vintage machine some sort of thread stand can help.
Good luck!