halflife flickers on the side.....g400 dual head

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when i play half life on opengl i get flickers on the left and right side of my tv..when i change to direct3d it doesnt give me the flicker but of course i want to play it in opengl..i have a matrox g400 dual head videp card 350amd k6-2 128megs of ram..and i use the clone feature to display on my tv(is there any other to show games on tv other than the clone, not like im having prob. w/ it im just curiose..)