[MAC] Current MAC mini playback issues

Hi,
I just got my mac mini today that im going to use as a steaming client.
I have the one with the 2.5Ghz Processor and ATI graphics card. I'm running OS X Lion and installed XBMC via dmg(version 10.1 Git:e9e9099 Mar9 2011).

I tried to play some files (h.264, mkv, 1080p, AC3 or DTS Audio) from my NAS via gigabit LAN and i get some random FPS slowdowns. I have to say I have a good eye regarding things like this.
Playback from local HD is not better either, the files Playback smoothly on my Desktop PC.

Any Suggestions?

Please define Smooth

The following article is a bit out of date regarding Lion but the core of the article could still be informative.

Please read the following article:
http://forum.xbmc.org/showthread.php?tid=87749

Thank you sooooo much man! It was one setting on my Samsing TV called Kino+ that made the movies stutter during certain scenes
Seems that Lion with 10.1 XBMC does VDA without framedrops.
But for some mkv'd i have to turn VDA off because its getting all pink and green, is the mkv not encoded right?

use mediainfo to check the mkv.

Does it play correct in in other mediaplayer. I could be that the mkv does not lie within the 4.1 specs