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power supply could be the cause of reboots during gaming, but it wouldn't be becuase of the voltage on the rails, it'd just be becuase the PSU it so small.
more then likely you're having the lockups and BSOD's becuase of temperatures, or a problem with your video drivers (or some other drives). probably vid drivers... update them.
All you know, really, is that your +12 rail is probably changing by 60 mV (millivolts, or 0.060 Volts).
MoBo voltage monitors are notorious for typically being off when it comes to absolute voltages, but typically very accurate when it comes to measuring relative voltages.
The only way to know, for sure, what your +12 rail is really doing is to measure it with a decent quality multimeter. Compare the reading of the MMeter with the readout in MBM or whatever you are using to read the voltage, and you'll have your correction factor. From that point, any relative changes are probably a good bet.
Lastly, the PCI spec for the +12 rail is +/- 5%, or 11.4 to 12.6 volts, so even if your readings are on the money (they aren't.... ) you are well within the PCI spec. While this may sound "sloppy" I know for a fact that most quality hardware manufacturers test their products to +/- 10% so they'll work even if the PSU isn't quite up to snuff.
in electronics -- nothing is exact. therefore, everything works within a 5% range unless otherwise specified. most resistors are 10%, etc... 12volts is just a ball park figure of what they want.