Nvidia Card Play Game Not Working

I have nVidia as per title, but when I want to play HD 1080p videos through VLC player I come into problems. It starts of 1-2 second then stops, while you can hear the sound, then plays the video for another 1-2 second and stops again, while you can hear the sound.When attempts to play the video, it jumps to where video should be together with the soudn you can see pixels and very slow movements and it stops again. Almost it seems that it is buffering the video while attempting to play it in the same time. I know that max resolution is 2560x1600 which is more then enough to 1080p HD videos. I have 17" monitor, Q6600, 4GB ram. how can I play 1080p HD file without having video output start stop. Went an downloaded trial version of CoreAVC and was able to play it through Windows media player