Internal network speed degrades to 10Mb/s

I have the ASUS RT-N66U using Tomato RAF Firmware v1.28.9014 MIPSR2-RAF-v1.3g K26 USB.

I dual-boot Win 7 and Fedora on my main computer that's hardwired into my router about 10 feet/3 meters away. My network connection randomly drops to 10Mb/s down from 1000Mb/s and occasionally it will be at 100.

How would I go about troubleshooting this?

Any input is greatly appreciated.

Thank you.

Both OS's do this?

One way is to use any logs either in router or computer.

Another is to half split. Swap something one one side or other.

Also give visual check.

Did it ever work?