SamyGO

Stuttering while streaming HD mkv files seem to be a common problem for the entire Samsung LED D-series.

I seems like on board CPU is to weak to perform h264 decoding and various picture enhancements simultaneously.
Nota Bene: Increasing bandwith in your LAN will not help.

One work-around is to disable some of the picture enhancement features such as Auto Motion Plus or Edge Enhancement (this solved it for me)
You'll find those features via Menu/Picture/Advanced Settings

On some models you are able to put the TV into "Game Mode" while streaming, which removes unnecessary processing.

On some models, stuttering problem has became more significant after fw upgrade and may also affect other sources as BluRay and NetFlix,
see following thread forums cnet com//7723-13973_102-539646.html (sorry about syntax, I am not allowed to post link)

do you use buggy UPNP? It could be the reason of all problems.

Buggy UPNP...are there some settings I should check? On my server or router?
I am using Serviio media server on 64bit Windows 7. Had a LE46B650 before and it never had any stuttering problems whatsoever on the very same LAN network.

upnp protocol is not fast enough. It would be great to know your opinion after NFS or Samba mounting. But sadly, we don`t have any root access on D5 series

Just for the records:
I discovered that some mkv:s still kept stuttering when watching them over DLNA/AllShare, even when all picture enhacement features are turned off.

So I took one of the h264 mkv:s with most problems and rewrapped it into MPEG2-TS stream. (I used the original video- and audio streams unchanged).
This new file works perfect when streaming it via AllShare from LAN connected PC. No stuttering at all! FF/Rew and position seek works!
Rewrapping kan easily be made with tools such as tsMuxer and FFMpeg

Conclusion: Firmware T-MST4DEUC ver 1024.1 as of 03 Aug, 2011,
which is used by LED TV:s D400x and D500x has a flaw which makes it stutter when streaming HD mkv movies with average bitrate > 1 Mbps. Problem is limited to matroska container only.

@Krobar: I am not sure this work-around will be of any help for you, i believe D6510 is based upon Arm CPU contrary to MStar CPU in D5005.

TSMuxer is abandonware, development has stalled.
Don't know if this can be helpful, but just to mention it in this context: MKVToolnix (Link) and MKVExtractGUI (Link)