Is the internal network card really broken?

[AzeofSpadez]

Hello everyone!

A friend of mine has recently bought a cheap used computer to play casual games with however the built in network card in the motherboard seems broken.
Both me and my friend have 100 mbit fiber connections.

When the computer is connected to his network it gets a 9.5-ish mbit/s down and up and it sometimes turns off for no good reason.
When the computer is connected to my network it gets about 90-100mbit/s down and 10 up (Which is what we both pay for) but the connection still disconnects every minute or so.

What has been done:

Reinstalled network drivers
Tried running without network drivers
Tried thousands of ethernet cables
Fiddled with Windows settings

What hasn't been done (Will probably try next)

Reinstalling Windows
Booting of a live Linux USB and testing connectivity there

I just want some feedback, what do you guys think? I think that if a network card truly is dead it shouldn't even connect to the internet and not receive full speed at one place and only sub 10mbit/s at another.
I have no clue what this is, any feedback is really appreciated

The motherboard is a Gigabyte GA-MA78GM-UD2H and the operating system is Windows 7 64 bit.

[fixx]

Sounds more like intermittent connection between the network chip and the motherboard chipset or intermittent power failure on the network chip. I would just buy a new LAN card and stop wasting more time on it because it sounds like you've tried everything possible already.

Worst case scenario is that the NIC is not only thing broken on the motherboard but just tip of the iceberg... You sure USB ports or HDDs arent showing any problems?