1080p mkv FPS decrease in Action Scene on Xbox360

Hey there

i've got a problem! I've tried to watch a HD Movie witch my Xbox360 and the WIndows Media Center. But in some Action Scenes the Video stopped for a Second or longer. If there is a quiet Scene the Video runs perfect.
So what can i do?
Ive just installed it without changing any options.

greez

My PC

Windows 7
AMD Athlon x2 5800+
Nvidia 9800gt
2gb Ram

and sorry for my english ;)