I was having problems with my phone connection, and a tech came out and INSTALLED a new modem. I had a single-band one, and they installed the new dual-band modem. Everything was fine for about a week, and then I moved to a new apartment.

I brought the new/current modem with me, and a technician came and installed the modem in my new apartment. Everything was fine for 11 days. On day 12, I was using the phone and internet when I lost my connection to both. The phone did not disconnect properly and even using my cell phone I got a busy signal when dialing the number I had been previously connected to. I performed a power cycle on the modem, and the issue was resolved.
 
I continued to have this issue repeatedly for several weeks. Sometimes the issue would be resolved with a power cycle, and sometimes it would not. Sometimes I do not have a connection for as long as 15 hours. Two technicians have been to my apartment and have not found any interference on the line. The signal history over the past 7 days has always been good.

I have tried the following methods of fixing the issue:

-A new single-band modem
-A new dual-band modem
-Four coaxial cables
-Plugging the modem's power cord into outlets on two separate CIRCUIT breakers
-Moving the modem to the OPPOSITE end of the room
-Operating the modem with nothing plugged into it (no phone or Ethernet cords, etc.)
-Connecting the modem directly to the coaxial cable in the wall as opposed to going through the switchplate
-Having the modem remotely reset by Comcast
-Pressing the reset button on the modem

The issue does not seem to happen at a specific time of day or a specific day of the week. It is not RELATED to the refrigerator or heat running, nor to weather conditions, as the problem occurs when it is not raining or windy. It can be fine for several days, or it can go out four times in one day.

Other Comcast customers in the area are not affected, as I can connect to the xfinitywifi network when my modem does not have a signal. The US/DS light flashes rapidly and the online light does not turn on. Comcast technicians on the phone say the modem shows as turned off or "does not register in the system" when it is in this state. It cannot be remotely reset in this state, as it is not receiving signals from Comcast.

The modem is a Cisco DPC3939 from Comcast, Hardware Revision 1.0. When it is not WORKING, it shows the following stats, some of which I am told are out of spec:

Downstream: SNR 29.807 dB; Power 2.600 dBmV
Upstream: Power Level 21.250 dBmV

However, when I have an internet connection, the stats are in spec. Comcast seems to be at a loss as to how to fix the problem. Is there anything else I can try myself?