I have no experience with your particular chip, but generally with overclocking your voltage is limited to mostly the amount of heat it produces. If your temps are fine you should be OK. You may want to look up the max specs on your CPU though. I believe the 2500k max safe voltage is 1.5v. At 1.39 you should be good as long as temps are not that high.
The Asus X79 boards push those voltages too high on auto, at least my RIVE does. They may be as high as 1.4/1.4/2.1 respectively which is far too high and can actually cause instability and will severely degrade the IMC/MCH. Set them manually to 1.2/1.2/2.0 and see if that improves stability
so i had look at my at voltages vccsa was 1.015 /cpuvtt 1.066 / cpupll 1.8 and i have set them to 1.2/1.2/2.0 prime crashed at beginning
then i got them back to auto and just now noticed software warning that vcore gone up to 2.004v!
so i went back to bios and switched of offset ill have a go with prime again
i got this AI Suite II instaled and this is what i found in one of the files:
[08/16/2012 at 02:13 pm] +12V 0.768 Abnormal
[08/16/2012 at 02:13 pm] +12V 12.288 Normal
[08/16/2012 at 08:59 pm] +3.3V 4.080 Abnormal
[08/16/2012 at 08:59 pm] +3.3V 3.344 Normal
[08/16/2012 at 11:38 pm] Vcore 2.044 Abnormal
[08/16/2012 at 11:38 pm] Vcore 1.382 Normal