The same thing happened with my Brother PQ1500S. Every time it happened, the bobbin tension was the problem. I know you said you checked that, but just sharing my experience. I wound another bobbin to test out the theory, and then it worked perfect. I'll never know why the other one didn't work that time. Some unknowable mishap when winding the bobbin the first time. So my recommendation is to win another bobbin even if the first one looks fine, start over from scratch by re-threading the machine, and see if that helps. I know it's really frustrating! You have my sympathy!