rmvb play back very jerky video

I use NitoTV’s Mplayer to play back RMVB files

I have different file size rmvb.
For the smaller file size ones, ( up to 200MB), it can pay smoothly,
however, when I tried to play the 500MB ones, only audio is flowing smoothly,
the Video becomes very jerky, frame rate becomes so slow.
like every 7-10 sec, it refreshes the screen video, very slow.

Any idea how to fix this ?
I see you can add more argument to the Mplayer executable,

anyone has some expierence in this ?