There is a test circuit on the "digital audio" generation of G4's that ramps the fan to full speed at startup. If this detection circuit is faulty it may detect that there is too much power being drawn on the system. Since the 001 gets it's power from the CPU, having the interface plugged in at startup could trigger the circuit to run the fan at a slower speed or not at all. At the time of this writing, all cases where Apple replaced the motherboard and fan, the issue was resolved.