If you have tension issues that appear on the bottom, it's often the top tension that's off. I'm wondering if you have a bit of fluff stuck in the tension disks which is keeping them from applying appropriate tension. use a sturdy thread to floss between them, and when you thread your machine make sure that the presser foot is up until the you've threaded the whole thing and are ready to put it thru the needle. when the foot is up, there is no tension on the disks so the thread can seat properly.