[nznog] Windows 2000/2003 Strangeness

Go gently on me this is probably considered off topic but I can think of nowhere else to post this hardware background : I have 2 windows boxes and a Linux box connected to my internal LAN and to the internet, all 3 have 2 nic's problem : the windows boxes disappear from the internet intermittently for 2 mins or so every now and then and suddenly come back, there is nothing in the event logs' the boxes just disappear for a bit. however the Linux box stays visible all the time. all 3 machines are connected via the same router. I can see nothing wrong with any of the servers and this is making me pull my hair out, any thoughts welcome Cheers Geraint