I'm not sure if this is the reason but might be better than nothing:
It might be because of overheat protection - computers usually shut down when components reach a high temperature to prevent hardware damage (it's a setting from bios).
Most probably it's from the Video Card or the CPU - they might overheat while rendering the videos, so make sure these won't reach high temperatures. If it isn't from this...well I don't know...probably the Video Card is failing.
Its because your graphics card is counting days. Its going to worn out soon. If you want to extend its life for some more time kindly deactivate the graphics booster software like gamebooster from your system. try to avoid streaming videos of High quality. Miniumize graphics usage as less as possible.