I had the same issue, and had mine replaced back in October. I was told (by a service technician at the Microsoft Store) that this happens when you constantly carry the device in a bag/briefcase and you only close the lid, which puts it in sleep mode. Because it's an "always connected" device, sometimes it will still get emails, updates, notifications in the background. I was told that when the device performs lots of background things, the processor is working and there's no way for the heat to properly dissipate because the vents are all blocked when it's being carried in a bag/briefcase.
It was suggested to actually shut down the Surface Book before putting it in a bag. I was able to get Microsoft to exchange it for me. They sent me an identical model through the mail and I returned the defective one. I didn't have to pay anything; they just put a hold on my credit card for the value until they received the defective item back.
I perform a shut down of my computer each night and it's only in a bag when I travel which isn't very often. I bought my computer at a military base, but I plan on taking it to the Microsoft Store and see if they can help me. I'll post an update after I go to the store. Thanks for your advice.