well i apologize for underestimating your skill set but the only things you've stated towards solving the problem yourself aren't exactly detailed or numerous.
and maybe change your name :)
oxide54's reply doesn't make sense because I deleted my post that wasn't very complimentary towards him.
I popped into the school on Friday and one of the machines on the curric network was exhibiting the same behaviour :(
Just what I need - 28 brand new desktops with dodgy network interfaces :(
You'd think by now this would be one thing that just worked on a computer :(
given you said it worked fine booting off usb / iso for a linux distro, its got likely its the windows drivers?
but i used to have problems running one of our servers using the HP supplied drivers it would randomly just not ping certain hosts on the network and I mean random, i ended up re-formatting and using the built in server 2008 r2 drivers and never had a single problem since.
How are your switche ports set up? Auto negotiate?
Do your switches have STP configured on all your ports?
Some nics cannot sync sometimes with some switches with certain configurations and this can cause conflicts whereupon the switch ports actually shutdown or the nics slow down to virtually no speed throughput at all.
If as you say a curriculum machine is showing signs of this also it could be a switch port config problem?
Another theory is that all those 28 machines have the same batch of nics in and they could be from a bad batch of manufacture, only way to tell is to install another nic of a different manufacture.
Good luck my friend :)