Recently I discovered that when I download a new game I can only play for about 20 minutes before the monitor goes to sleep. This does not happen with games I have installed on my computer – only new downloads.
It's happening because of overheating of CPU or GPU.Clean off all the dust and then try playing games to see any improvement.This will definitely help you.
See the below article -
How to Clean the Inside of a Computer
Maybe, but if it dosnt happen with installed games then unlikely.
Check your power settings in the Control Panel and turn off any power saving settings and try again, it will give us a clue.
download a game? which kind? flash....
try this microsoft solution