Hi, almost certainly your controller. The problem with these old wired controllers is kids used to wrap the cable around the controller over and over again which in turn created breaks and loose connections inside the wire. You could lop of part of the cable but without knowing exactly where the break is make things difficult. My advice is simple......try a different controller. I am sure thats all it is as you have said you have cleaned you controller with good ole isopropyl. Failing that its just cold solder points on your megadrives controller input ports. Simply reflow them But I am sure its just your controller.