I have three media devices that can stream from two PC servers, all connected to the DIR-657 via Ethernet cables and a switch. All devices experienced the same issue with the original DIR-657 firmware 1.01 and now 1.02 beta. I tried a factory reset, which was a nuisance because it cleared all my reserved DHCP addresses, and even then it did not solve the problem. I tried toggling UPnP, SPI, and so on, all with no effect. All devices connect successfully to the Internet; the issue affects only streaming across the LAN. I asked D-Link tech support (web, e-mail, and phone) but they could not resolve this issue.
The problem did not occur with my old D-Link DI-524, only with the new DIR-657.
The problem is, if I turn off a media device, and later turn it back on, it does not find the PC servers until I reboot the DIR-657. Switching the DIR-657 off and then on, or unplugging it and plugging it in again, does not solve the problem. When I reboot the DIR-657, the media devices can see the PC servers and stream continuously for days, until I turn off a media device, then I'm back to the problem.
My first hypothesis was, the DIR-657 power-saving features might power down ethernet ports when the connected devices are switched off, but I don't know why power off&on would not solve that. My next hypothesis is, the Tools->System->Reboot process causes the DIR-657 to send some information that the media devices need, and that the DIR-657 does not send otherwise.