Sounds to me like it might be the sensor. You know it works via a tiny laser light shining across the bobbin to determine the thread fill. I have 2 Brother ULT machines that drove me crazy saying my bobbin was almost empty when they were over half filled. I adjusted the laser warning & now I can go almost to the 'empty' bobbin. So if you have cleaned well and are sure no lint is in that area, it might be that the sensor is faulty. But I'd try vacuuming that area and do another cleaning before rushing off to the repair shop. Hope you find the cause--it is very annoying. I remember only once it happening to me like that--can't remember now what the cause was for the machine stopping ---oh yes, seems it tho't the top thread was broken! Guess I didn't have it threaded well. Anyway--good luck!