I have a Pfaff 332 which is the machine I learned on when I was a little boy. My mother had it for 30 years and I've had it for almost that long. The cordset has been replaced but it has never ever done that and I've lived in 4 different states in old houses as well as new and renovated ones. I smell B.S.
The machine could be wired wrong but "old wires with new wires" being somehow incompatible is bunk and I would never use that electrician again. That just doesn't pass the smell test.