HDTVs max out at 60Hz, yes even those LCD HDTV claiming 120Hz and 240Hz as well. 120Hz HDTVs operates differently than a 120Hz PC monitor.
Just to clarify, watching a movie on the laptop results in no issues, but when watching it on the plasma HDTV the video is choppy.
I would try updating or re-installing the nVidia drivers. Also try borrowing a HDMI cable just in case your cable is defective.