Try threading bottom thread through little hole on bobbin case if it has one. My bernina works better with some threads when I do that,
completely re thread top and bottom from scratch. Sometimes this works with my singer, no idea why .
Do you have same thread top and bottom? Neutralise everything and go slow increasing and decreasing top then bottom until you find a good stitch. This I would only do as an emergency as it takes ages moving one at a time and keeping note.

I have gone back and reread all of the posts, and the one thing no one has mentioned, when suddenly having this kind of problem, especially when starting a new project, is have you checked your needle size? Sometimes we forget about 'fixing the seam in the jeans' moments, when we need a heavier needle, and then when we go back to the usual fabric without changing the needle, you can have the problems that you describe.

Sounds simple, but I've done it enough to have finally learned...make sure your presser foot is down!

I had that problem awhile ago and the problem was polyester thread on the top and cotton thread in the bobbin (or vise-versa, can't remember for sure), but when I put the same thread on top and in on the bobbin, it all worked out fine.

My machine tells me when I forget to put the foot down. I hate to tell you how often I hear that tone...lol!