Just for info, here is a video of the problem which I originally sent to Sony...
That clarifies things a little. In all this testing I assume that you have been using the Sony to pull media from MC. Since you are using DLNA, and pulling media, I assume your server is not located near the TV. Do you have a laptop or something you can use as a MC Client to try pushing media to the TV, instead of pulling? I don't think that will solve your issue, but it would be interesting to see if pushed media works straight off or not. It may give Andrew a hint as to what is happening as well.
Also, I can't tell from that video if the Sony is being flooded with data, or starved of it. It could be that the MC DLNA server isn't responding quick enough (or too quickly) when communication is via a 1Gbps connection, but at 100Mbps it does. Since the TV only supports 100Mbps, but the PC supports 1Gbps, there may be some speed negotiation going on every time the Sony tries to pull media. That causes enough of a delay that the Sony decides playback isn't going to work. When you try multiple times to play media, there is enough time for the speed negotiation to complete. Or maybe multiple attempts "force" the DLNA server to communicate at 100Mbps. Or the network card. Hence, if you restrict MC to 100Mbps, no speed negotiation is required, and playback works first time. I'm guessing a bit here, as I'm no network/DLNA expert.
If the above is correct, it isn't a Sony problem, and is more likely to be a MC DLNA server problem. Which takes you right back to the start. Although maybe it is a combination of both, as the Sony probably should wait for the MC DLNA server to switch speeds.... Or it could still be a purely network layer issue. Although I have always found Intel NICs to be
the best for standards compatibility, so it should know the Sony is only 100Mbps, and communicate at that speed. Unless effectively a new network connection is created every time playback is initiated.
I think you need a network expert. But you know what you are doing. Maybe you could use WireShark or similar to analyse what is happening at the server NIC, in terms of connections and speeds? You may even be able to see the speed negotiation happening, or the new connection being made to the Sony. I don't know.
Finally, just because why not, have you tried turning on the "WMC compatible" flag on the MC DLNA server? That may fix some idiosyncrasies that the Sony expects.