Internet Drops and Intermittent Modem Reboots

A few weeks ago, we started noticing our internet would lose connection for 3-4 minutes whenever the air conditioning switched on. We have lived in our home for approximately seven years, and we have never experienced this issue, so we thought it was the router (an Asus RT-AC68u that replaced an RT-AC66R a few months ago) , so we switched the router out several times, but the problem continued.

Over the last few days, the problem started getting worse, and we would notice the internet would cut out even if we kept the AC turned off. Yesterday, I finally took a closer look and realized it was the cable modem that seemed to be power cycling. It is an SB6141 that must be 5-7 years old. I tried different things - plugging the modem to a different electrical outlet, making sure the power cable and coax cable were not loose, power cycling the modem, making sure the modem was directly connected to the coax coming out of the wall (v. through the splitter that the original tech put in ([back when we had cable TV]), etc., but the problem has persisted. 

At this point, I am thinking if it is one of three things: (1) an ISP issue, (2) the cable modem is failing, or (3) the power supply is failing. I think this because, one of the logs indicate: "Cable Modem Reboot due to power reset." Also, the problem has been especially bad the last few days, when our area has experienced higher than normal temperatures.

Attached are the logs showing the modem's signal strength, as well as the logs.

https://imgur.com/kylWTXlhttps://imgur.com/lMFGILD