But we digress, OP's issue, like he stated, is about how they are powering the problem rig. If you want to split hairs as to which diodes you looked at and what the readings of your multimeter were when you were looking to undervolt, then I'll leave you to it.
The OP raised voltage setting, all adseb and I pointed out was that is does not actually change the core voltage. If the S3 was not well past it's sell by date I would persist in the discussion, however as it's now academic I will call it a Day on the S3 Voltage setting discussion.
If anyone else want's to chip in with their experiences of software voltage setting on an S3 then please do so. I for one have had my fill of being called a liar in every other paragraph...
Rich
If you do not want to be called a liar, do not lie, simple.
The datasheet is quite clear on the recomended voltage setting for different frequencies, so persisting with your lies that the voltage setting has no use, while saying in the next breath that you do not know what the chip actually does with the settings from cgminer, simply cements your assertions as pure lies (and which liar wouldn't have their fill if they were found out?).
The voltage setting, like I stated, will not resurect an inadequately powered S3, so the OP has to ensure they are powering their rig adequately before even being concerned about the voltage setting. Matching the frequency they are running at with the recomended voltage setting (or a touch higer) will definitely reduce HW and / or improve hash speed.
EDIT:
Here's one example, properly powered with a Dell N750P-S0 server PSU with 4 custom made 16AWG 6 pin PCI-e cables, that's been running for a couple of weeks with the voltage setting matched to that recomended for the frequency in the datasheet.
I usually let these run for months on end without restarting save for when the PSU's fans die (common with these PSU's running in a shed in the garden) and the PSU shuts down.