Hi!
I have been having this problem for years with my PS3 and I thought it was a PS3 problem, but now I have a Samsung TV ES5500 with DLNA and I have the same problem: about 20 minutes playing a film and then it disconnects. It's pretty annoying!!
In this case it is interesting because when it disconnects you can not find the media server in the list of DLNAs devices but Samsung TV has a "recent videos" list and then you can select again the video you were playing and it works!! And the the disconnection does not happen again, I guess because you are not really connected.
It's seems like the media server had to send some package saying "I'm alive" but it doesn't and then some timeout...
My device is a Terastation Duo, 2TB, RAID 1, Firmware 1.55, and it was recently reset.
Cheers