Problem playing mkv files

[cvrak]

Hi,

I use bs player for a long time, mostly with 720p mkv files.
Sometimes the video runs slower than audio, sometimes the video freezes and after 3-4 secs it plays normally.
Yesterday I tried to play a 1080p mkv file, but the fps was 10-12.
When I tried to play again 720p files, the video plays very slowly about 20-21 fps.
Which renderer is best for my case? Any other settings I have to adjust?
I use an Acer Aspire 8930 Notebook, with Nvidia 9600M GT, 4GB Ram, Windows Vista Home Edition.
Do I have to install any external codecs pack, like CCCP?

Pls advice
Chris

PS The strange thing is that some months ago, I played mkv 1080p files without any problem .....