chromic :
safe max voltage for fx chips is 1.55v.
And actually fx8320 and 8350 have the samw binning, for example ive seen 8320 hit 5.0 ghz with <1.5v and 8350 struggle to go past 4.5
and i cant even boot to windows @ 5.0 with any voltage.
They don't actually have the same binning. The 8320s wouldn't exist if they did. they make mistakes while binning CPUs sometimes and a really good one goes in the not so good pile and the opposite happens too. Most people who have 8320s will tell you that they hit the same frequency as the 8350 using an 8320 but only with higher voltage than the 8350 would need. This isn't always, because as I said the binning process isn't flawless but most of the time this happens.
The reason the 8320s exist though is because of the binning process, they are just slightly under cut 8350s.
Back to your particular CPU though, if it isn't stable under stress test then you really need to drop the Ghz a little or raise voltage that last little bit otherwise you will do harm faster to your CPU.