Hard to say, sam, for certain. The symptom you describe occurs whether the power is too low or too high! However, if your GameCube has gotten this way all by itself, then the power is too low, so you have to turn it UP.

To turn up the power, turn the screw anti-clockwise. Turn it only a little at a time (like; no more than 5 degrees) and see if that fixes it. Repeat until it works or until you give up (it *could* still be a different problem).