WD Link to a NAS

Hello I am using a WDTV 1TB Hub.  I have it linked to a Drobo FS 5 bay NAS via a  

RT-N56U Wireless N Dual Band Gigabit Router linked by LAN.  I have movies encoded as MKVs and at times i am noticing that video is choppy at times.  There seems to be no rhyme or reason for this as there are times when the network is really busy (downloading, moving files) and there are no issues with playback of even HD video files (the WDTV and NAS are prioritized for network traffic), at othertimes the network is not in use and even low def files payback in a very choppy way.  I am curious where i can get info to help resolve this issue.

Is it always the same files, or do some files play fine one day and choppy another day?