I had the same thing happen to me with a Brother machine. It is a computerized machine. I was using the foot pedal and I was chin piecing and after about ten minutes or so, it just took off all by itself! The only way to stop it was to shut it off or unplug it.
I took it in to the shop who sent it to the dealer and they could not recreate it. It had happened to me several times. The dealer and the guy that owns the shop seemed to think it may be related to the use of the foot pedal. It seems that (maybe) by unplugging the machine and and unplugging the foot pedal it resets something and it hasn't done it again since I picked it up.
The guy at the shop said next time it happens to unplug the machine (leave it turned on) and to bring it to him. He said maybe he would be able to see what was happening that way? I don't know.
Do you know someone with the same or similar machine that would let you borrow a foot pedal to try? It may be your foot pedal and not a computer issue?
Good luck.