I just upgraded to the Xfinity Gigabit Internet. I have a Arris SB8200 cable modem and tested the connection through ethernet port in the cable modem and my connection is maxing out at 375 Mbps on speed test. I have talked with Xfinity customer service twice and they say everything is fine, but on my end im not seeing the speeds I should be getting. Just a FYI, I also use to have the X1 modem on the 600Mbps plan but wasnt able to get those numbers either.
FWIW, this may not be the direct cause of your issue, but it should be addressed regardless as it may be another problem just waiting to happen.
The upstream power is on the high side and it may be intermittently fluctuating even higher to out of spec levels. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, and latency problems.
In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test
If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.