Note that if you're running an internet speed test, your downstream speed will be affected by your upstream path as well. This is TCP/IP, after all, so downstream packets must be requested and acknowledged via the upstream path. If your upstream packets are struggling to get through, your downstream througput will be low no matter how well the downstream path itself is performing. Comcast could be correct that your downstream path is capable of 1.2Gbps, but that may not be the real issue.
A common indication of upstream problems is ranging failures. Look for the message "No Ranging Response received - T3 time-out;" in your log file. If this happens more than, say, once a day (not that it should be happening at all), you probably have a serious upstream problem. You should also look at your upstream power levels. If the CMTS is requesting that your modem transmit in the neighborhood of 48dBmV or above, there's a good chance a CM1150V simply isn't going to work. The fundamental reason for this is that the CM1150V is a DOCSIS 3.1-compliant modem, but Comcast is running a non-DOCSIS-compliant cable plant. You can see more about this here.
If this is indeed your situation, no amount of rebooting the modem or jumping through Comcast's other mindless troubleshooting hoops is going to solve the problem.
