Arris sb8200

When I first turn on my pc, settings say 1000mbps and the modem is blinking green. After a little while, the speed dips, settings say 100mbps and the modem is blinking orange. Wifi is faster than ethernet. I swapped the blue cat 6 cable, I am now directly connected to the modem without a router. The modem is four years old. I was told to check cable signal levels, but don't know how.