So, if I understand correctly your monitor shuts itself off in the middle of gameplay? I have a similar issue and I've since underclocked my GPU and downgraded my graphics in the games that cause this issue.
Could be that your GPU is overheating. Hwmonitor will show your GPU temps. Maybe have a look at those during your gaming sessions to see how high it goes.
MY GTS250 does the same thing. My monitor just turns off. PC is still on but I have to hit the restart button. Can't alt-tab or ctrl-alt-del.
That's my best guess.