Keep changing the tension until the machine is sewing as it should is my advice. I just got the 8900 pretty recently and have had to adjust the tension for FMQing depending on the weight of the fabric, the thread and the batting. You have a great machine. Don't give up! Have you taken everything out of the bobbin area and cleaned it lately? I do that every few bobbin changes. Lint can build up pretty quickly. Also, it might be time to bring it to your dealer to be serviced.