Thats irony for you
If it was working fine when linc sold it to you, I'd say its just been worn down through use- you might get lucky with neu saying they will warrenty it, you may not. The other motors you have look like good canditates for 5 or 6s lipo, just gear accordingly- the medusa may suffer the same issue in future though if it has the wires coming out the back, so its worth adding some insulation to the wires either way- I dont like that they are tinned their whole length either, makes adjusting them kinda hard.