capped at exactly 10mbit for LAN but internet is fine at 40mbit?

How is it possible that computer to computer transfers are exactly 10mbit/sec but my internet connection is working fine at 40mbits with the same computers and network adapters? One computer is asking me to activate because I put the CPU back at stock from an overclock. That couldn't be it, right?

I have two Windows 7 computers and a linux NAS server, all gigabit, and each device on the network is connected with only one network adapter. The system was working flawlessly with gigabit speeds for a year, and all I did was replace two motherboards.

By disabling/reenabling the network adapater in windows or restarting the router, I can sometimes get the LAN speed to behave like real gigabit and do sustained 800-900 mbit transfers, but it soon gets eratic and sometimes pauses for a few seconds.

I can eclipse 10mbit/sec with my internet connection with both computers 100% of the time, but for some reason computer->computer and nas->computer transfers are capped at 10mbit/sec most of the time.

edit: I may have fixed it by resetting everything on the network. I was pretty sure I already did that, though. Very strange.

try swapping round a few NICS (if you have them installed on your board or.. wires which sometimes can be the problem (but very rare to be the cause but worhth checking) maybe the motherboard coould be the problems if you didnt have any problems before they were replaced... are you sure the new boards have gigabit ethernet... sounds like they dont if your network is now being capped at 10mb... if they would be 1 gigabit etheret on the board then i cant see that being the problem.... maybe post a pic from device manager of network adapters, controllers etc