Arris TM1602AP2 Modem, directly connected to PC. Speedtesting on speedtest.net - Spectrum Ohio server, as well as several local servers. Zip code is 40489. Internet service only, previously had tv service. Splitters have been eliminated from the lines, and the line to my modem is connected to the line coming from underground with a coax coupler. Have heard of cellular causing interference, so for giggles my carrier is Verizon.
Your case is a prime example of the problems encountered by the vast majority of people...slow speeds, picture pixelation, dropped connections, etc...that are almost always caused by a coax issue of some kind. I'm glad you were able to find the problem and fix it.