hmmmm, this sounds familiar.....I had this problem - found on two of my machines...gbpvr server included one of the windows updates i did at the end of August screwed my tcpip stack - under a bit of load (< 10%) it would stop responding to the network for a couple of minutes... Putting in the second NIC card didn't help (thought it might have been hardware)
Never did figure out exactly which update caused it, but have narrowed it down to 5....a mate of mine suddenly noticed the same problem so I gave him a list of updates on my newly fixed machine in the hope that we can figure out which one it was.....
AMD FM1 x4 A8-3870 Llano, Gigabyte GA-A75-D3H, 4 Gb DDR3 Ram, Seagate 500 Gb, Seagate 1500 Gb, HVR3000 x 2 (DVB-S), Nova-T 500 (DVB-T), USB-Uirt, SPDIF -> Yamaha RX-V540, ATI HD6550 (OnBoard) > Samsung LA46A650 TV (HDMI), Harmony Remote 525
Windows 7 Ultimate x64 SP1, NPVR 2.4.3, AC3Filter, 1 x Popcorn Hour A-100, 1 x AppleTV with Plex