AMDs temp sensors are known to be not very accurate, especially with lower temperature (can happen to intel also, tho much less frequently)
i had an athlon 64 x2, a 5000+... with my WC, i was able to bring down to -10 degrees... at least the sensors said that... they were about 20 degrees off, because my ambient was about 10c at this point, i benched that night, in winter, and simply opened the windows,to bring down the temps, whilst using a thermometer to determine my ambient.
all in all i could say: you need a temperature sensor, thats mounted into the HS or the bottom of your cooler/block, to get real temperature readings... most Sensors just give out Temperatures "over the thumb"... meaning its well enough for ordinary safety,just to be able to say whether its too hot. but not for real OCing, nor personal confidence
just my 2 cents on the most needed, and most worst supported safety feature. the Sensors.
In my Opinion, there should be an array of sensors for each part of the die, each single mosfet, and each ram IC... but... who asks me?
P.S.
you cant get under ambient, not with air, not with water. if you want to use conventional methods, you can only use a bong cooler, to get minimal below room temperature... after that, you can only use a chiller, a thermoelectric Element (Peltier) standalone, or a phase change. (LN2 and He2 not to mention, they speak for themselves ;-))
You probablly won't even need to add voltage for it to boot.
