Office internet blipping every 3 days

Hi All,

I've had an issue ongoing for a year since our business internet was installed, and the ISP is seemingly unable to determine where the issue lay, of course they are pointing fingers at my environment. I'd appreciate your thoughts on what the hell else I could look at troubleshooting here. The topology is a Rogers cable modem (Coda-4582) 1000/50 static IP configuration, to a Sophos XG firewall. The Sophos alerts when the gateway goes down, and this happens approx every 3 to 3.5 days. It seems to be a full power cycle, as Rogers reported that the online time and registered time both reset when this event happens. The rack is powered by a Tripplite line interactive full sine wave UPS - and no other hardware is having power issues. I have escalated to "Business Management Office" and the tech is suggesting power issues on my side.

I've tried the following with no change in the behavior.

Modem replaced twice (now on the 3rd unit)

Powering the modem directly from 15A circuit without UPS

The unit is passively cooled and I measured the heatsink temp at 85 degrees Celsius. I suspected it may have been overheating so I put a 120mm fan on the side, dropped temp to 40.

Disconnected the modem from my infrastructure and it continues to power cycle with only it's coax and power attached. (Thankfully I have redundant ISPs or I wouldn't have been able to wait the 3 days to test this).

onsite ISP tech thought the signal looked a bit low, so replaced the coax from the primary floor cable to the distribution point, and from the distribution point to our office. Signal is good now.

Suspicions I have left are - perhaps there is some strange AC power harmonics and the UPS can't filter out, and the power supply in the modem is of poor quality and can't handle it. Or perhaps the modem is trying to reach some provisioning or NTP server in the ISP environment and can't, so it reboots itself on a set interval +random offset value.

I've suggested that last idea to Rogers several times but they keep dismissing me. Funnily enough I've had this happen with my residential connection, where the modem would drop out every 24 hours, and it took 3 tech visits, truck roll to fix lines on the street, and lots of calls by yours truly, for them to realize it was the modem failing to reach the NTP server, and so would kick itself every 24 hours.