Game shuts down in the middle of it

On all my full-screen games I play, I always experience
the same problem. During the middle of play, the game
either minimizes or ends for no reason at all. Any
suggestions for fixing or even finding the source of this
problem?

Also, I don't know if this has anything to do with the
same problem, but if I don't continuously keep the window
active e.g. continuously type in Word, sometimes the
window will become 'inactive' e.g. the title bar turns
grey and I must click on that window to start typing there
again...

Thanks,
Rich