What are system specs? Full brand and model name of graphic card and PSU?
Have you added the graphic card recently? You did not have this issue before?
If that's the GeForce GT 710, that 300W you mentioned should be the PSU recommended by Nvidia not the cards power draw.
That card I think is passively cooled and doesn't even have a fan. It's draw maybe somewhere near 30-40W and is fed by the PCIE slot and doesn't need a PCIE power connector either.
If that PSU you have is a good and reliable 600W unit is more than enough for the card. Although I'm guessing it's not that good a 600W PSU, as only delivering 140W on the 12V is a sign. Maybe the PSU is old.
The 12 volt rail being 11.5 amps it can deliver 140W (138W to be precise) if it's in proper working condition but apparently it's not.
If indeed it's a power issue, which may very well be, you need to get a good brand'model PSU. We can suggest PSUs after we know system specs and, therefore, it's requirements.