I have had ongoing issues with maintain gigabit speed. Multiple trips by techs etc and replacing modems, cabling and actually removing an amplifier. The one common thread seems to have been related to provisioning.
When I lose gig speed I drop down to what appears extreme+ and downstream pegs right around 650-660 but I generally still see gigabit layer uploads (around 42mbps). Losing gig speed has happened (looking in modem and router (pfSense)) when the data harvester loads int he modem logs and/or a power cycle of the modem. The recent fix (had worked the last two times since early March) was a reprovision.
So now I have fought the battle again, call CS, yes me again, read the notes in my account and kindly reprovision the modem. Have to go through all the steps so they can get to the point where they can reprovision. Unfortunately, this time, the reprovision doesn't work. Get escalated to Level 2, same thing, reprovision doesn't work. Get a call this morning from a very unhelpful more senior tech that said my issue is just due to network congestion. Said signals were fine and be happy with 600 Mbps. I am kicking myself for not getting his info and if anyone from Comcast cares I am sure they can find out who it was.
SO, now to the fix. I tried a factory reset of the modem. Guess what, back to gig speed.
So for some of us out there who are more IT inclined, have had m,ultiple expeicirnces with the same issue, don't accept "it's network congestion" if you think something else is at play. I want to determine if this is a fault of the modem software or what. Maybe it is time to go back to owning (like a did with Surfboards in the past until I needed an MTA for phone).
EDIT: XB6 modem...
FWIW here's a retail Gigabit speed capable eMTA;
This issue was even before this current modem - so lost faith in the XB6 in general. CS says no XB7's yet but we know that isn't the case...
So might grab an Arris but then recall the horrors of getting a customer owned modem working...
Well - the data harvester ran at 3:00am (looking in the modem logs) and I see the drop of connection in the router logs as well. Sure enough when I wake up back to my 650 Mbps "cap". Did a factory reset just now and wouldn't you know I have gigabit back again.
Real bummer in all this - my router died and had to cobble up a new box to run pfsense. And before anyone thinks it was the router. When I built the new router and first connected I was at 650 Mbps. Factory reset the modem and now where I should be (920-940 Mbps).
This is a pain if it indeed happens again tonight.
No surprise, happened again. 3:32am in the logs (so different than the last 2 nights). Speedtest this morning and back at 550Mbps. Factory reset the modem, change password, set to bridge mode and now it will only offer up a 10. IP (i.e., it isn't OUT of normal mode and won't go to bridge mode without a power cycle).
A power cycle I have learned will reset the modem speed. This happened 3 weeks ago when things were proper with the internet speed, had to move device to a different UPS, and then when I checked the speeds - back from 930 - 550-600.
Think a new modem is a guarantee for now though I have to believe there is a background process Xfinity runs in the middle of the night that is causing the issue. A new modem will help rule out the modem and then Xfinity might need to dig deeper into WTH is going on here... (unlikely).
Don't expect a retail modem to fix all your problems. I have the same problems with inconsistent download speeds (wired and wireless) using the XB7 in bridge mode. Download speeds are inconsistent even using the XB7 wired directly to my desktop. I tried the retail Arris T25 and the CM1150v first, but they had inconsistent download speeds AND keep dropping of the network, with a boatload of uncorrectable errors on all download channels. So, I'm sticking with renting the XB7. Your mileage may vary, however. I hope it does.
FYI, when bridged to a router with an address of 10.0.0.1, the XB7 IP address is 192.168.100.1. Unfortunately, every attempt to log in (with the correct password) is greeted with "Access Denied" and a 5 minute lockout after 3 attempts. Connecting the XB7 in bridge mode to the directly to the computer's ethernet port, however, allows me to successfully log in. Odd.
I finally ran a speed test in Windows 10 Safe Mode with Networking. Full gigabit speed. Rebooted into standard Windows 10 mode - about 350 mbps. I downloaded TCP Optimizer (free), ran it as an administrator, saved my current settings (in case something went wrong), optimized, and rebooted. I'm now getting full gigabit speeds. Your mileage my vary.