This happened to my Janome, which I had for about 20 years. It began to sew by it's self, very scary and dangerous! In my case it was a shorting out of the wires in the foot control pedal but I did not need to take my machine to get fixed just the foot pedal and cable to an electrician to get new wiring put in, once fixed it was perfect. The electrician said I was lucky I did not get an electric shock from the thing!
Gal