A client is reporting that his internet speed is high for several days at a time and then will intermittently slow down to about 1/4 speed. He's using a Spectrum cable modem. I think I heard him say his high speed was 260 mbps. The cable between the modem and the router is probably CAT5 and runs about 30 feet. I put a cable tester on the cable and it passes. Is there a reason this would happen?