+1 to ILL and almost +1 to Ratters. If the gun is well used and ILLusion's fix doesn't work, then it's the electrical safety that's failing. The selector plate connects two tabs to allow electrical current to pass through. The selector plate get's charred after a while, and through use more and more charring occurs until it won't connect the tabs any more. This is why many v3 gearboxes loose semi-auto after a while, but can still fire in full-auto.
You can try to clean/sand down the charring carefully, but you would still have to open the gun on a regular basis. Getting a low resistance/high temperature selector plate could work, but I recommend something better, cheaper, faster and more permanent: Solder the two tabs together. On many newer switches, it's even normal to have this bypass factory-made. Check out this totally stolen
picture for an explanation (ignore the text in it. It's for some other guide).