Monitor Turns Black While Playing Crossfire

Ok, heres the problem. While Im playing crossfire, everything works as well as usual. But for some reason, after a few games, my monitor suddenly turns black and the power button flashes on and off. It doesnt show anything afterwards and I cannot turn it back on and I have to restart my comp. It started to glitch like this recently. It never happened a month ago. Also, I didnt change my software/comp parts during this time. Also, Im sure it only happens with crossfire, so its not a problem with my computer. Moreover, I use skype with friends that i play crossfire with and when my monitor turns black (shuts down) I can still talk on skype, but i cant see anything and i think crossfire closed because i cant hear gun shots either. Please help if you know anything about this annoying problem.