If you floss the upper tensions, just make sure not to use anything that is *waxed*. You can use several strands of regular thread together, or you can use unwaxed dental floss.
Do you have any other sewing machines? I ask just in case it's possible that you are using the wrong bobbin in your machine. Often a machine will sew even if it's the wrong bobbin, but will create bizarre problems.
What machine is it? Have you removed the throat plate and cleaned the feed dogs?