Any computer nerds here? Hard reboots

[abaris]

So I bought a premade system last year.

AMD A8-6600K APU
8 Gigs DDR 3
Win 7 64

The only thing I did was to deactivate the built in videocard and to replace it with a Geforce GTX 650 ti.

These last few weeks I have hard system reboots when either watching web videos, such as Youtube or Vimeo and in games like Mass Effect 3 where there are cinematics. It only happens when videos are playing. Not during actual gameplay and not when the computer is simply running other programs.

I browsed the web extensively for an explanation and installed Speedfan to do some monitoring. Now I noticed that CPU temperature as well as Core temperature go wild when a video is played. From an idle of 42 degrees C (CPU) and 7 degrees C (Core), temperature rises to 70 CPU and 59 Core within three minutes. So, my guess is, at longer videos it performs an emergency shutdown because temperatures actually go to high.

GPU temperature on the other hand stays constant.

That's inexplainable to me, since simply playing a video shouldn't stress the CPU as much.