I have a Brother Innovis 4500. It did the same thing. The tech who fixed it told me he found an extra piece of thread in the upper threading unit. My machine has an automatic threading feature. I asked him how did he fix it...Duh! He said he took out the screw on the back of the machine...it's by the needle end on the upper part of the machine. The side cap should come off. The machine has been perfect ever since. My husband is very mechnical & will be the first one to touch my machine if it does this again. Good luck with yours.