Thank you WD_Guru for your insights. I agree, however, I did some tests and checks today.
The outcome was rather unspectacular.
I checked the video bandwidth of a normal HD video. It is around 35 Mbit per second MAX, plus sound of about 1 Mbps. That is well in the range of a G network with good connection - around 50 Mb/s for a 108 Mb/s connection. Well, if hte G network connection is able to achieve this throughput etc.
In my case, I have checked a Cisco WRT610N router with a Cisco WET610N WiFi bridge and a Belkin 100 Mb/s hub.
I have tried the full path, a direct cable connection from the router to the hub and from the router to the WD TV Live.
I fogot to mention, that I am using a WHS to distribute the data.
After checking all these different set - ups and also changed the firmware on the WRT610N to the DD-WRT version, I can say - no change!
All kinds of set-up showed the same result - choppy performance of HD movies. Now, I am believing that there is a problem of the source, the WHS and the hard disk performance. This is something, I have found toaday and I would like to share with you. It seems, the network is not very stable however, I also tried this on a PC and got the same results. Next thing I am going to do is to change my old Samsung 501 HDs to newer ones. May be this helps. However, I am still puzzled.