LG PC monitor problem can anyone help?

yesterday i was starting my pc when i turned the screen on it starts to blink on and off in a constant way and stopped when the user screen appeared the monitor is LG L1760TR and graphic card ATI 4850. i have windows xp and 7 on 2 separated partition of the hard disk. i tried to shutdown everything and just turn the screen on btw the screen is plugged into a ups not directly to the wall and also it blinks so i dont know what's happening. every time i start the pc it blinks till it gets to the user screen then everything is good . any idea? thx