I agree, it's likely tension or needle related then, but my guess is tension, and I want to say upper... I know you've already checked this, but the spool isn't catching the thread in that little "slit" that you can use to make sure the spool doesn't unravel when it's off the machine, right? I think that usually breaks at the top, hence my question.
How does the bobbin tension feel? If you pull on the thread how much resistance would you say it has? There should be "some" but only a little more than barely there.
Do the same for the top tension. If you thread the machine up to the needle (but not through it), put the presser foot down, turn the dial to 0, when you pull on the thread it should have "barely perceptible" tension. If you turn the tension dial up, the tension should increase. If it's really tight at 0, or it tightens extremely fast compared to the numbers on the tension assembly you might need to adjust the assembly to be more "Accurate".
There's no burr on the needle plate is there? A needle strike can cause that on the hook or the needle plate.