Duemilanove won't switch to external power

For my next project, I need the analog-to-digital converter (ADC) on the arduino to be as accurate as possible, and thus I need the ADC reference voltage to be accurate. After some quick multimeter probing I find that the external power regulator outputs a rock solid 5.00v, while the USB power tends to fluctuate at around 5.05-5.15v. As a result, I would like the arduino to use the external power from the regulator.

From what I've read, the arduino should automatically switch to external power once the voltage is above ~6v. That doesn't seem to be the case for me :'(. As long as the USB connector is plugged in the 5v line will always read the 5.05-5.15v from the USB input, even when the external power is connected.

I took a look at the arduino hardware schematic and the power selection circuitry appears to be working fine. When the external power voltage exceeds 7v, pin 7 on the LM358 goes high (measured 3.83v) and feeds into the NDT2955 mosfet gate. But regardless of whether the mosfet is on or off the 5v rail still reads 5.05-5.15v. When the USB plug is removed, the voltage will return to a stable 5.00v. However, I need the USB for serial communication and can't remove it.

Any help would be appreciated, Spectral

It sounds like you FET is Fu**ed and acting like a short.

You can always remove it completely.

I'm looking at the Duemilanove schematic at http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf Isn't the P-FET's source and drain backwards from what it should be?

FET = [u]F[/u]lame [u]E[/u]mitting [u]T[/u]ransistor