A few shots in the dark. Is the setting for your WIFI channel Auto? Could it be that the assigned channel is getting overloaded? By disconnecting and reconnecting the internet cable change the assigned channel? Could it be the modem/ISP. Power cycle the modem and see if that helps. Bypass the router to see it it the PC slows down. Is the modem at the "end of life" or "no longer supported" per your ISP. Is the firmware in your modem up to date?
Let us know what works.

The WAN channel is set to AUTO. I actually don't care if the wifi is slow. I rarely use it. I am trying to figure out why the gigabit LAN to internet is slow. I have already tried disabling the Wifi a few times.

My cable modem is a Technicolor T4350 with a 75mbps connection. It is less than a year old and still under warranty. Although it suffers from the Puma 6 chipset jitter problems. If I can prove it is broken, I can exchange it for free.

Bypassing the router resolves the internet speed problem. Restarting the cable modem also solves the problem temporarily for 1-3 days.

Here are connections:
3xLAN: Two desktops and one Obihai voip adapter
4xWAN: Two cell phones, one smart thermostat and one IPad. These devices are mostly idle.


Thanks for the help. I am looking for ways to troubleshoot the router.

1) I suspect it could be the router getting hammered as my voip adapter tries connecting to google voice every 4 seconds. I have since disabled this.
2) I suspect it could be the cable signal strength as I see it fluctuate to -7mdb. But that wouldn't explain why bypassing the router works unless I got lucky.
3) I suspect that it is either my router or cable modem overheating as they do get hot.
4) I saw authentication errors on my Android phone. I later found out there is a glitch in which the SSID cannot be the same for 2.4ghz and 5ghz bands. I have since made the two SSIDs different.

If bypassing the router works okay it would indicate it is the router. I would try the following:
1) Replace the cable between the modem and the router.
2) If heat is a suspect, move it away from other heat sources, make sure it is on a hard surface, and add a small fan or place where it gets some air flow.
3) If the VIOP can be connected between the modem and router, try that.
4) Borrow (or buy a cheap router at a yard sale) and try it out temporarily. (I have an old Linksys "G" router that I keep as back up. Very slow but works)
5) Reset the router to factory and do not set up WIFI until you test to see if it fails. I do not think it is the WIFI however, if it does not fail, set up the SSID password to a different value and attach each WIFI device one at a time.
6) Change the NAT BOOST setting
7) Check to see if IPV6 is working ( https://test-ipv6.com/). If not, set it up.
8) Verify the firmware is up to date.

Let up know it that helps.

Another thought...
Check your PCs for malware and such. Run Malwarebytes, CCleaner, your antivirus scans, device checker on your ethernet hardware connection.
Also use a different browser. I had Firefox leak and slow down my computer over time. I used the Firefox Reset and it helped for a while. Switched to Chrome and no more issue. I suspect it was caused by one of the Firefox extensions/add-ons I was using.