choppy playback 1080p on HDTV via DVI/HDMI

hi,

i have a mac pro dual quad 2,26GHz, 6GB RAM, 2x NVIDIA GeForce GT120 graphics.

for editing of HD material in final cut pro, i use 2 Dell displays 24" 1920x1200 on one graphics card for timeline etc, plus an LG full HD TV via a Blackmagic Intensity Pro video card as video output. in this setup, i can playback 1080i50, 1080i60 and 1080p (1080p24) from final cut pro flawlessly. smooth motion in any frame rate.

now my problem: accessing the same LG HDTV set via my second graphics card (ie as a 3rd computer monitor) for playback of 1080p24 movies (mkv), by converting the DVI out to HDMI. whatever frame rate (1080i or 1080p, 24 or 25 or 30 or 50 or 60 Hz) - the video playback looks choppy when playing back 1080p on quicktime player 7 or 8 or VLC. strange enough, when adjusting the graphics card's output frame rate to 24 Hz (which should match best the movie's 23,98p), result is even choppier than on 30 Hz output. on 30 Hz, playback is choppy mainly due to pulldown (24 frames to 30 frames).

question: how can i get smooth playback of 1080p24 - either via my graphics card OR the blackmagic video card?

thank you!