Question : Network Connection Switches from 1Gbps to 100Mbps for no Reason

    I am having a weird network problem where my NIC will just switch from 1Gbps to 100Mbps, and I cannot figure out why.  I've tried running different programs while watching the networking tab of Task Manager, and I cannot figure out what is making it switch; I've also tried putting my PC into Sleep mode, but that doesn't seem to be making it revert back to 100Mbps.  I'm not sure when it happens, but ever since I noticed it happening I check Task Manager on a regular basis, and then all of a sudden for no reason my Gigabit NIC will say 100Mbs instead of the normal 1Gbps.
     What's even more weird about the whole thing is that I have two Gb NICs, and the same thing will happen to either one of them.  I only have one of them plugged in at a time, and normally they both say 1Gbps in Task Manager, and of course only one of them says connected because I only have one of them plugged in.  The weird part of it is that once the NIC that I have plugged in reverts back to 100Mbps the other unplugged NIC will stay at 1Gbps; however, if I unplug the CAT 6 cable from the now 100Mbps port and plug it into the other port then the other post then says 100Mbps and the original NIC switches back to 1Gbps.
     I know it sounds like it is the router, but I have tried a couple of different brands of Gb routers, and I have also tried installing Vista a couple of time t fix the problem, but the end result is the same; somehow the NIC still reverts back to 100Mbps.  Also, once the NIC reverts to 100Mbps the option to set the "Speed & Duplex" setting to "1000 Mbps Full Duplex" disappears in the Advanced tab in the hardware's properties in Device Manager, and the highest choice available at that point is only "100 Mbps Full Duplex".  Additionally, if I take the same CAT 6 cable and plug it into my media server's Gb NIC the server still has 1Gbps speed, so it cannot be the router.
     Thanks in advance for the help, Jon

Answer : Network Connection Switches from 1Gbps to 100Mbps for no Reason

Ideally you want to use the PCI-E NIC, it has more hardware reasources allocated to it so it performs marginally better than the PCI NIC.

If a driver issue doesn't resolve the problem, you might want to check your cabling (not the actual cable), but use the cable tester app in the BIOS, if it comes back with errors on the cable, your NIC might be physically damaged in a way that is preventing 1Gbps negotiation.