it doesn't work that way.
every chip is different, and will require different voltages to stabilize their clockspeeds.
but there is no way to make a CPU stable at lower volts. its not to do with temperatures, or the cooler.
the reason a voltage increase is required is because an increased clockspeed reduces the time between cycles for the system to get the data needed. increasing the voltage strengthens the signal so the data can be fetched more easily.
so, if it si unstable at a certain voltage, you are stick with it. as long as temps are good though, there is no need to lower the voltage.