LAN connection to cable modem reverts to 100mbps

We have a gigabit network with a 500 mbps cable internet connection. I noticed recently that our connection was topping out at 100mbps.

I logged in to the cable modem to find that the LAN interface is showing up as 100mbps. After simply unplugging and replugging in the lan cable, it jumps back to 1,000 mbps and everything works fine for a while sometimes a few hours, sometimes a few minutes. Then it will be back at 100 mbps. The cable modem plugs into a wall jack that runs about 30 feet away to my switch and router.

Is this likely a poor quality cabling connection or is it possibly something wrong with the cable modem? How do you troubleshoot something like this?