I have had this happen. It is such a helpless feeling. Let me explain what is causing mine, if I can. Perhaps it will help. Somehow the foot pedal cord that goes up to my machine gets caught under the pedal when I am pushing my foot down. When I take my foot off, the cord is sort of stuck there and keeps running. If I reach down and pull the cord out of the foot pedal, it stops and is working fine again.
This always catches me surprise and it doesn't happen often. But maybe my experience will be something like what is happening to you.
Oops, I just saw your last post. You don't need my answer now....
. but that is okay. I hope your machine keeps running.
Dina