Ubuntu Multimedia :: MKV Files (both 720p And 400p) Playback On 10.04

I have some trouble in playing mkv files (both 720p and 400p) on Ubuntu. I had no problem whatsoever playing the same files when I still had Windows, so I'm assuming this is not an hardware problem. I have an HP Pavillion laptop, an nVidia graphic card with proprietary drivers for Ubuntu, dual core AMD processor, and Ubuntu 10.04 64bit installed. Many other applications are actually way faster, now, than with Windows, as that was just a 32bit version. Nevertheless, I have problems. Totem is usually slow, and generates lags between video and audio. I used MPlayer (with Gnome GUI), and it was ok with some files, but with some others (can't find any particular logic, maybe it's due to different encoding methods) it will just start, not show anything, NOT CLOSE, and enter in some kind of stall. The CPU overheats and I'm forced to reboot to stop it.

(It's not due to folders too crowded with mkv files... I've tried to take the files out of their folders, they still give trouble). Finally, VLC kinda works, but still won't allow me to jump in between the file. With some files it just doesn't jump. With a file I acquired recently, it jumps to exactly HALF of the time I actually asked for the jump (Like, I ask to jump at 42 min., it jumps at 21 min.). Chapter selection won't work either.