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
Jon BredensteinerProject ManagerAsked:
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maninblac1Commented:
Marvell, the maker of your NIC has released many many updates for the Yukon NIC, usually a new one every 2 weeks or so.

The ASUS driver is 8.56.6.3, Marvell is currently releasing 10.51.3.3
You can get the update here, see if it helps.
http://www.marvell.com/drivers/driverDisplay.do?dId=101&pId=39
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Jon BredensteinerProject ManagerAuthor Commented:
Ok, I just updated my Marvell driver to v10.51.3.3, but I'm guessing it won't help since the same thang happens with the Realtek NIC too when I swich the LAN cable over; hopefully it does workl though.  Thanks for your help, Jon

I didn't mention it above, but here are my NICs:
1)      Realtek RTL8169/8110 Family PCI Gigabit Ethernet NIC (NDIS 6.0)
2)      Marvell Yukon 88E8056 PCI-E Gigabit Ethernet Controller
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maninblac1Commented:
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.
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Keith AlabasterEnterprise ArchitectCommented:
Have you forced the nics to operate at the speed you want or are you using autosense at both ends? Autosense at both ends of a link has never been the optimal approach and a number of the Gigabit cards do not handle it very well.
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Jon BredensteinerProject ManagerAuthor Commented:
    Thanks for the suggestions guys...  I will try the cable tester app in the BIOS when I get home tonight.  I installed the updated bios for the Marvell NIC, which is the one I normally use as it is the PCI-E card, and I haven't experienced any problems since the update, but I haven't used my PC that much either since then.  I'm still doubting that it was/is the driver since it happens with both the Marvell and the Realtak NICs.  I currently have both of the NICs set to 1Gbps, but I have not set the router to that speed, so I will look into that too when I get home.  Thanks again for the help, Jon
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Keith AlabasterEnterprise ArchitectCommented:
Thank you :)
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Jon BredensteinerProject ManagerAuthor Commented:
    Thanks guys for all the help.  In the end, I just reimaged my PC back to a point before I installed Roxio Easy Media Creator 11 using Acronis True Image 11, and everything works now; however, I split the points up between you guys, as you did provide helpful troubleshooting ideas.  Thanks again, Jon
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