I had the same problem with the first rMBP. I used Intel's power gadget and could see the CPU clocking down to .8 GHz after a few minutes of Google hangouts. A

I had the same problem with the first rMBP. I used Intel's power gadget and could see the CPU clocking down to .8 GHz after a few minutes of Google hangouts. After a clean install and still having the problem Applecare let me schedule a pickup for my laptop. Had it back in less than a week and then everything was fine. So the less was to gather the evidence with utilities (such as the Intel power gadget), and then once you know it is hardware problem you can just keep pushing until you get a replacement but feel perfectly justified since you have evidence.