The Ethernet connection is normal, but it disconnects itself after a while and then reconnects after a second. How to solve this problem?

The issue of an Ethernet connection that is initially stable but then experiences brief, cyclical disconnections and rapid reconnections is typically a symptom of an underlying fault in the hardware, software, or network configuration, rather than a complete failure of the link. The core diagnostic principle is to systematically isolate the point of failure, as the problem could originate from the local computer's hardware or drivers, the Ethernet cable itself, the upstream networking equipment like the router or switch, or even external interference. A stable initial connection rules out gross misconfiguration, but the intermittent drops point to a component degrading under load, a power-saving feature misfiring, or a persistent but low-level physical layer fault that triggers the link to renegotiate.

Begin the investigation at the most immediate physical layer. Replace the Ethernet cable with a known-good, high-quality cable, as internal wire damage or a failing connector can cause intermittent signal loss that manifests as these brief drops. Simultaneously, test the connection by plugging the computer into a different port on your router or switch. If the problem persists, try connecting a different device to the same cable and port; if the other device remains stable, the fault is localized to your original computer. This step is crucial for determining whether the issue is host-specific or network-wide. Within the host computer, outdated, corrupted, or buggy network interface card (NIC) drivers are a prime suspect. Access the Device Manager on Windows or equivalent system profiler on other operating systems, completely uninstall the current NIC driver, and install the latest version obtained directly from the motherboard or NIC manufacturer's website, not through Windows Update. Furthermore, delve into the advanced properties of the NIC in your operating system's device settings and disable any power management options that allow the computer to turn off the device to save energy, as these features are a common culprit for cyclical disconnections.

If the problem transcends a single computer and affects multiple devices on the same port or switch, the fault likely resides in the network infrastructure. A failing router, switch, or modem can develop faulty ports or overheating issues that cause periodic resets. Power cycling the router and modem is a basic but necessary step to clear any transient software glitches. For a more persistent issue, consult the administrative logs of your router for error messages around the time of disconnections, which may indicate WAN-side problems from your Internet Service Provider or internal DHCP conflicts. In enterprise or advanced home settings, a network loop caused by misconfigured cabling or switch settings can create broadcast storms that destabilize connections. A final, more technical possibility involves Ethernet auto-negotiation or duplex mismatch issues, especially if older and newer equipment are mixed. Manually setting the NIC and switch port to the same speed and duplex setting (e.g., 1 Gbps Full Duplex) can resolve instability caused by failed negotiation, though auto-negotiation is generally preferred when it functions correctly. The resolution path is therefore defined by isolation: methodically testing each segment of the connection from the computer outward will identify the failing component, whether it requires a cable replacement, a driver update, a hardware reset, or the replacement of the networking device itself.