Could be an electrical problem with the foot pedal or the motor. Did it smell hot? Hopefully not. Go back to the yard sale and ask the former owner if they had this problem. If they are honest they will tell you why they sold a machine that is so highly recommended and used.
You definitely need expert advice and help with this but a call to Janome will be the start of the process.
Every journey starts with a single step.
Caro J.