Display random blank

Hi guys please help me, i have Digital alliance hd5850 with newest driver and if i played games, random blank screen occur about 2-3 second and goes normal again its happen randomly sometimes about 5 times a minute, its so annoying, this happen if i use hdmi cable, but if i use my bottom dvi connector (my vga has 2 dvi connector) its even not going back if blank occur, but if i change to top dvi connector all goes normal, whats happen to my vga? or is it my monitor?