Ethernet Adapter disconnects after large amount of data passes through to an outside network

Solution 1:

It turns out the Intel 1225-V Network driver chip has issues involving faulty timings for 2.5G operation. (The delay between packets is wrong). This causes intermittent periods of high packet loss. See this report for example.

The reason for your card only experiencing issues when transferring large amounts of data might be power saving options automatically changing its speed.

The problem may or may not manifest depending on the hardware in the Switch or Router connected to your computer. I.e. it may allow the Intel card's out-of-spec behaviour or reject some of its packets. Or it might detect the faulty behaviour and automatically negotiate a lower speed, fixing the problem for you.

Fix

To fix your problems, go to Control Panel > Network and Internet > Adapter Settings, or Right-click your network connection icon located in the bottom-right of the taskbar, select Open network center, then click Modify adapter settings (to get to the screen linked in the Question).

Select the intel I225-V. Right-click and select the bottom option Properties. Click the Configure button below the name of the controller. This should open a new tabbed modal window. Open the Advanced tab. Scroll to the Speed & Duplex setting. It will probably be set to Autoconfigured. Instead manually force it to 1.0Gbps Full Duplex. (If you have really old network equipment or use Cat4 cable then you might need to select a lower speed option such as 100Mbps Full Duplex).

Source

Intel released a bulletin to OEMs (PC suppliers) where they recommended pre-configuring the network driver for 1Gbit operation. Since this is a hardware issue: they are releasing a 'stepping' fix, meaning cards manufactured after Q2 2020, that is past 1 Sept. 2020, are supposedly not going to have this problem anymore. Some users are still reporting problems. This means you won't be able to fix it by updating either drivers or firmware. Instead you may want to apply for a refund or replacement, as after all the card won't operate properly at the advertised speed of 2.5 Gbit. If you do, be sure to inform the place where you purchased of the manufacturing defects or they might supply you with another faulty card.