Ethernet cable not detected after new service installed

Hello,
I switched from DSL to cable about a week ago, I have two desktops connected via ethernet cable to the modem.
One works fine, the second stays connected for 5 seconds, disconnected for 5 seconds, connected, again, etc until the system is on for 45 minutes or so and then it stays connected. I feel like it is probably a setting and that the system is trying to detect something during that time. Problem is I have no idea where to start and Comcast is totally useless. I have already replaced the cable because that's what they said the issue was, it wasn't.
Any help would be greatly appreciated!
btw, all other computers, including wireless, are working fine.
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  1. what OS is on the computer that has the connection issue?
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