I am running a fresh install of Ubuntu 12.04 LTS on my desktop with a Gigabyte GA-Z68A-D3H-B3 motherboard, which (unfortunately) uses the Realtek RTL8111E LAN chip. It's a well documented issue that the network cuts in and out or runs very slowly on Linux—I'm experiencing this as well.
I downloaded the official Realtek driver (r8168-8.035.00.tar.bz2) and successfully ran "sudo ./autorun.sh".
If I run this script while my network is non-functional, it will fix it. However, inevitably, a couple minutes later, the network will cut out again until I re-run the script.
I'm not sure exactly what's going on. Realtek did not respond to my support request, I have tried numerous online tutorials for older versions of the driver/kernel, and I attempted to make the switchmod script work. I didn't have success with any of them. (Note: Unlike this guy, my motherboard does not support IOMMU, so that's not a viable solution either.)
If someone could help offer suggestions or troubleshoot this issue, I would really appreciate it! Thanks!