Good to know the old BIOS works. Thanks for sharing that.
I have two G3258s that we use at school for teaching basic overclocking. One runs at 4.2Ghz at 1.310v and the other runs at 4.5Ghz at 1.355V. That's the best speed and lowest stable voltage we have managed to get.
Neither of them are especially good chips, the first is far below average, and the second is about average.
Every chip is an individual. I'd want to know what voltage IS stable first, before I started tuning.
Our procedure is to start with brute force and ignorance, setting the multiplier and then the voltage to get it stable, and only afterwards fiddling and tuning to bring the voltage down.