PC restarts when playing a game then a black screen..

Greetings everyone, i have a bit of a weird issue with my computer.
when i'm doing nothing major, like surfing the webn watching youtube, everything is pretty normal. but when i play a video game, like Need For Speed Most Wanted, or Elsword, everything goes fine for 10-20 minutes, then i can notice that frame rate dropped a bit, as if the compuer is having a hard time dealing with the game. and before you know it, the pc restarts. What's more is, once it shuts down, the dislplay no longer works and the screen says "no signal". so what i do is wait for it to complete booting till i hear the windows welcome sound, then i shut it down and wait for it a couple of minutes . thankfully after this, when i start the pc again, the display works again.
I'm kind of lost, any help ?
my pc is Acer aspire M1920, 2GB RAM, cpu : pentoum dual core 3.2ghz and the graphic card Nvidia GeForce 210(1gb )