Computer Crashes When Left Idle If Being Used Extensively

This is a weird problem that showed up in my laptop over the past few months, but I'm just now heading here to try to get some insight on the problem. So when I first got this computer about two and a half years ago, it could edit/render/encode HD videos with little sweat. Now, it can sorta still do that. The only quaint problem is that it tends to crash if I go AFK for about 10 minutes or so while it is doing so. I can sit while rendering a video and browse the internet with no problem--I don't even really feel the laptop getting very warm, let alone crashing. However if I am to leave it alone while I go to do something else, I always come back to it turned off. When I reboot, I get the typical "Windows did not shut down correctly" spiel.