Monitor signal disappears

The problem is that during demanding moments in games, the monitor goes out and the cooler of the video card slows down. In this case, the sound works for a while. This condition continues until the power is turned off. The temperature of the card does not exceed 60 degrees. I don’t understand at all what can be done about it. Video card Gigabyte ati radeon hd 6850. Power supply unit for 550w. help me please