DIAGNOSIS This sounds like a failing Ethernet cable, a bad port on either the router or your PC's network card, or an unstable Ethernet negotiation setting.
FIX Start with the simplest fix: swap out the Ethernet cable for a known-good Cat5e or Cat6 cable. If the problem stops, you found it. If it continues, try plugging the cable into a different LAN port (1-4) on the GT-AX6000. This rules out a single faulty port on the router.
IF THAT DIDN'T WORK The auto-negotiation between your PC and the router might be glitchy. Log into the router, go to LAN -> Switch Control. Find the specific LAN port you're using and try manually setting the speed, like "1Gbps Full Duplex," instead of leaving it on "Auto." You may need to adjust your PC's network adapter settings to match.
HEADS UP If you've ruled out the cable and router port, the issue could be with your PC's network adapter driver. Update the driver from your PC manufacturer's website or the Intel/Realtek site. A failing network card in the PC itself is also a possibility.