DIAGNOSIS This points to a problem specific to that Ethernet port or the traffic management settings affecting that wired connection. It could be a faulty cable, a bad port, or QoS/bandwidth control rules accidentally applied to that PC's MAC address.
FIX First, try a different Ethernet cable. Then, try a different LAN port on the back of the AXE75. In the router interface, go to Advanced > QoS > QoS Settings and ensure it's disabled to test. Also, check Advanced > Security > Access Control to make sure your PC isn't being blocked or throttled by a schedule.
IF THAT DIDN'T WORK There's a feature called 'IGMP Snooping' that can sometimes cause latency for gaming traffic. Go to Advanced > Network > Internet > IGMP Snooping and try disabling it. Also, in Advanced > System Tools > System Parameters, ensure 'NAT Boost' is enabled—hardware acceleration is critical for low-latency wired connections.
HEADS UP If you've assigned a static IP to your gaming PC, make sure it's outside the DHCP range (found in Advanced > Network > DHCP Server) to avoid an IP address conflict, which can cause exactly this kind of intermittent lag.