Why Does My Monitor Turn Off After Watching Youtube Videos

Whenever I watch 240p and 480p YouTube videos one after another, my monitor would turn off.

Someday my PC shutdown automatically when I was watching YouTube videos. My CPU usage was not beyond 75%. The temperature of the components were:

CPU - 68°C

AUX - 74°C

HDD - 56°C

I was using Firefox 4 and the total RAM usage was 770MB. Such things even happens even when I play lightweight games like Tinker. Two months ago, I had replaced the heat paste of CPU by white coloured heat paste as I had never replaced the paste for 4 years.

Is it graphics related or heat related problem or both?