Sewing machines can be head scratchers, but it’s just a matter of sorting it out.
Has it ever sewed correctly since you’ve owned it? If so, when someone is starting out, most odd stitching issues are traced back to the thread path being incorrect or thread tension up top or bobbin tension being too light or too hard.
With any troubleshooting it’s always a good idea to replace the needle of correct size and orientation - slightly bent or improperly installed needles have cause many headaches.
If it’s never sewed correctly, that might be why it was put up for sale. Open up the access door and make an up close video of it making a stitch turning it by hand until the problem pops up. If the hook doesnt cleanly get behind the upper thread weird things happen.
Dull, damaged or improperly adjusted hook or needle timing are a deeper issue - many adjustments aren’t intuitive, so don’t adjust anything you aren’t sure of. As the needle reaches the bottom of its travel and begins to rise the thickness of a nickel, the hook point should be at the middle of the needle if it’s properly timed. The gap between the point of the hook and the needle should be a small gap or even lightly touching, but NOT deflecting the needle. Never adjust needle to hook gap with a used needle.
Unfortunately, old machines have usually been adjusted half ass by a number of people over the years and it pays to work through them to eliminate as many issues as possible - it’s just part of the joy of owning old machines.