I just registered after many moons of lurking as I may have a solution!
I have the same Intel NIC and there is a known issue with this device and Windows 8.
Basically, it can suddenly be detected as a different model (82579LM) and stop loading the driver, thus rendering the connection useless...
There is, thankfully, a firmware fix to correct this issue:
Intel download page with info
It's very straightforward: just follow the relevant instructions, reboot as requested and I think you'll find the problem is resolved.
Hope that helps!