Problem with internet connection (great details provided!)?

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I have been having a problem with my cable internet connection for about 4 months now. When my internet is up in running, I get around 20Mbps DL and 1Mbps UL, very fast. Though occasionally it will cut out for long periods of time, and I was told by the cable company that there was barely a signal from the coax cable. I have a motorola modem, and I notice that the PC Activity light seems to blink less frequently and consistently when the internet is down, perhaps meaning something. I do not have a router or wireless in my house. I used to switch the one ethernet cable between two computers occasionally to do stuff. Could this cause an IP problem? It's so strange because it occasionally will work, and be incredibly fast, but more often than not it doesnt work.

Thank you for your time reading my questions, and please give detailed answers!! Don't tell me to call the cable company, it doesnt work! lol