I will say this once so we can all understand it!
IT HAS NOTHING TO DO WITH YOUR SYSTEM RAM - Don't even think about replacing it. Games use VRAM not RAM, I expect you are using less than 3GB while gaming - task manager will show you.
Black screen sounds like the driver crashing and failing to re-initialise fast enough causing your games to crash. I'm guessing that Asus DCUII is factory OCd? Use EVGA PrecisionX to 'downclock' it back to original nVidia specs and go play some games (albeit at slightly lower frame rates). My bet is that it will no longer crash.
Run Heavan at max settings on a permanent loop for a few hours (while at DCU2 factory shipped clocks) and I imagine it will crash more than once per 10hrs.
If no crashes at NVIDIA STOCK, its time to claim on Asus's loverly 3yr warranty.
P.S Also just saw your extra bit of info in topic, could very well be heat related and thermal throttleing. So time to get GPU-z out and set the temp sensor to show max and update in back ground.