DIAGNOSIS This points to a problem with the Ethernet link negotiation between your PC's network card and the specific router port you're using.
FIX First, try a different LAN port (one of the four yellow ports) on the router. If that works, the original port might have a physical fault. If the problem follows the cable, replace the Ethernet cable—cat5e or cat6 is fine. If it follows the PC, go into your PC's network adapter settings (in Windows Device Manager) and look for "Energy Efficient Ethernet" or "Green Ethernet" settings. Disable them. Also, try forcing the link speed to 1.0 Gbps Full Duplex instead of Auto-negotiation.
IF THAT DIDN'T WORK Log into the router and go to LAN > Switch Control. Ensure "Jumbo Frame" is disabled unless you specifically set it up. Also, try disabling "NAT Acceleration" under LAN > NAT Acceleration. While this may reduce peak throughput, it can stabilize a flaky connection for some setups.
HEADS UP Some third-party antivirus or "gaming" network optimizer software can interfere with the Ethernet driver and cause drops. Try temporarily disabling such software to test.