Go the gigabyte webpage and find your mother board and see if you have the latest driver. To find the driver that you have. Right click on my computer go to properties and then click on hardware. Then device manager. Then click on Network Adapters then right click on that properties. Go to the driver tab and see what one you have, see if there is a newer version on the website. Depending on the router you have how to change the MTU will be in different spot. If it is linksys it is under advance settings i believe.
you should be able to find the MTU under Home and then the WAN tab.
make sure that you have all of the firmware for your router also, you should atleast have version 2.36, there was a reconnecting issue that got resolved in that version.