Hey sorry for the late reply, but i managed to snag a week away from work.
Finally i managed to get my hands on a quad core version of the thin client and immediatly tried it with the used Intel Quad NIC that kept resetting the dual core version.
As i thought it works flawlessy under heavy load, with all 4 ports used. This basically confirm that whatever the issue is, was tied to some specific hardware shenanigans on the dual core revision of the thin client. It's also a lot cooler due to the frequency being lower i guess, the other was skyrocketing to 80C without a fan onto it, while this sits at around 49-55C.
At this point i may try a newer dual nic on the dual core version just so i can use it on some other appliance which requires less LANs and if i do, i will report the results here, but as a general advice i would just say to avoid the other two versions and stick to the smaller one.
HF guys!