Your machine is almost identical to a Tin Lizzie..when I had that issue, the connection that holds the wire into the back of the machine was loose..they wear out over time, especially if you have any pull on your cable..I tried to find a pic, but couldn't. It's a round connection with pins in it, goes into your power box at the back of your machine.
If you don't see what I mean, I'll take a pic of mine for you..but bear in mind..I am recovering from a broken leg and it will be a real hassle to get myself behind my machine right now!