Ping/latency spikes seemingly out of nowhere on ethernet
Hello, around January 1st, I have noticed during online gaming sessions that I would occasionally, and sometimes frequently, get ping spikes for a few seconds when before going back to normal. For example, in League of Legends, I would get around 17ms-18ms at all times, while now it fluctuates between 17ms-30ms and in a few minutes get hit by high ping anywhere between 50ms-150ms for a few seconds. In other games that are very sensitive to packet loss, it's very noticeable and almost unplayable.
I don't remember doing or installing/changing anything that may have introduced this issue.
Here are my downstream/upstream signal levels: https://imgur.com/a/zzBGpjK
I pinged google.com and 22.214.171.124, normally and in safe mode, and the ping would fluctuate anywhere between 9ms to 35ms on average while sometimes getting up to 100ms-150ms request timed out (and even "general failure"? sometimes), pretty much exactly like how I've noticed it while playing games. Packet loss seemed to average around 2%-6% before and after changing things that I listed below, but I don't think these small variances had anything to do with those changes.
I have Windows 10 version 20H2, Xfinity with 1Gbps/down and 40mbps/up (my download and upload speeds are completely fine), have a Netgear CM1000 modem and Netgear R7800 router (I never had QoS enabled). I have tried almost everything under the sun that I can think of, which I will attempt to put here:
>Power cycled both modem and router for 15 seconds and again for 15 minutes
>Restarted modem through Xfinity Assistant
>Tightened coax connection on modem and outside
>Factory reset both modem and router
>Plugged modem and router directly to outlet instead of surge protector
>Dusted modem and router
>Chatted with Xfinity customer support, and they weren't able to see anything wrong, signals looked good and offered a tech visit, but held off as to see if I can't resolve the issue on my own
>Changed ethernet cables connecting modem to router and router to desktop (they were both either some form of Cat5 or Cat6, don't really remember but download and upload stayed exactly the same)
>Changed DNS to Cloudflare and GoogleDNS
>Tinkered with ethernet controller advanced settings, such as disabling offload settings, disabling eco settings, changing receive/transmit buffers to lowest/highest settings, speed/duplex, and the other settings as well
>I have a Z490 board and I was heavily convinced it might be an issue with the I225-V chipset, so I installed a Realtek NIC to test that theory, but it didn't seem to be the case
>Tested wireless on desktop, but similar/worse ping fluctuations occurred
>Plugged my modem directly into my desktop to see if it could be a router issue, but playing games and doing ping tests showed similar fluctuations, including timed out requests followed by high ping. I even tested using my laptop using an ethernet adapter, first connected through router, then just the modem, but ended up with similar results
I probably did many other things that I don't remember off the top of my head, but I don't believe they were super significant. At this point, I have a feeling it's an issue regarding a possible faulty modem. It could even be ISP related, or cabling issue, but my signal levels do seem good, so would that really be possible? I sadly don't have a spare modem to test this but if there is no other reasonable culprit, I can try ordering another CM1000 or similar/better 1Gbps modem and just return it if my issues persist. I'm up for any suggestions. Thanks!