I stumbled across this problem and solution but didn't see anything posted about it on Google, so I post it hoping it'll help someone else.

So I replaced my old cable modem after it's Ethernet port stopped working. New modem should make everything better, right? Nope. The new modem sync'd up and showed near-perfect signal levels & stats. BUT the modem would lose sync and reboot itself every 7-15 minutes.

Solution: I moved the cable modem 6 feet away from the router. 6+ hours now without a single error in the logs, no reboots, no dropouts, no packet loss.

It looks like the RF shielding in the modem is minimal and not good enough to handle the RF emitted by the router. I had decided to rearrange the equipment rack when I was changing modems and that's when I moved the new modem next to the router.