Car will suddenly just die?

Your battery shouldn't be the problem, 'cause the battery's only really used when the engine's starting or off. When the engine's running, the alternator gives you your power.

It IS possible that a loose connection on one of the alternator wires is shorting out, cutting all your power. I'd say check those connections first, then trace the wires and see if any of them are broken or shorting out.

My battery usually dies at about 3 years, so your battery may be getting old. If I remember my Mustang wiring right, the battery does complete the power circuit. So for example, if you disconnect the battery the car will also die. I do NOT suggest you do this, it causes a surge in the electrical system and can damage the computer.

It couldn't hurt to check your battery cables and make sure they're free of corrosion. Make sure the corrosion hasn't seeped down the wires either.