I'm a gigabit customer in the Denver area utilizing a Comcast-provided Arris TG3482G modem in bridge mode. Approximately every other day, I notice that my speed test results (via speedtest.xfinity.com) dip from their usual 800Mbps to sub-100Mbps. After digging around, I noticed that when this occurs, the LAN/Ethernet port's connection speed value (Gateway -> Hardware -> LAN) is 100Mbps. After resetting my modem, the value returns to 1000Mbps and my speed test results return to normal. Again, this behavior repeats roughly every other day, and a reboot seems to temporarily resolve the issue. Please let me know what additional information is required to investigate this issue. Thanks in advance.
You said the Gateway is set to bridge mode, do you have your own wireless router or switch? are you running speed test hardwired into the comcast modem directly? Can your computer handle a GIG? Go to the network setting and check that you LAN port is set to auto or 1.0Gb. If on auto set to 1.0 Gb. It more sounds like a issue with your computer unless you have tested with multiple device that support GIG.
The modem is in bridge mode, yes. I have an ASUS RT-AC66U wireless router. I am testing hardwired and have a fairly decent understanding of networking and CPE (I work for another ISP). The decrease in speedtest results isn't the primary issue-- it's the fact that the LAN interface, after a period of time, renegotiates to 100Mbps instead of 1000Mbps. I've tried swapping known, good CAT6 Ethernet cables in and out to rule out cabling issue to no avail. After resetting the modem, the LAN interface's link speed reports correctly as 1000Mbps until some time later when it renegotiates to 100Mbps.. Resetting my wireless router doesn't resolve the issue, leading me to believe it's a problem with the modem.
Possibly a firmware bug. There are boatloads of posts here from the past indicating many many firmware / chipset bugs with the Comcast supplied gateway devices.....
I would suggest a direct connect of your PC to the modem and see if it's still happening after a period of time. I suspect the issue is the router and not the modem. I had to get rid of my Asus after upgrading to Gig due to it being a bottleneck capping off speeds around 250M when a direct connect yielded a proper 900+.