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)
http://forums.gbpvr.com/showthread.php?t=29601
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