[cleeds]

Computers. They're great when they work and a nightmare when they don't. Many of us have endured problems similar to yours and I'm sorry for your anguish.

The first suspect in your system is the cable modem. If it's from Comcast/Infinity, dump it in favor of buying your own. That will save you a monthly fee and put all of your network equipment in your direct control. If a new modem doesn't resolve the problem, you have to move past the guesswork and fingerpointing and undertake a thorough, complete diagnostic evaluation. Anything less is a prescription for further issues. Based on everything you've said, the problem is likely in your ethernet wiring. Given that you apparently have 350 feet of ethernet, it could be anywhere in your network.

I'd bring in an independent third party to assess your issue. There are computer techs that provide this service and are capable of determining beyond a doubt where the fault is - a good one will be thorough and not stop until he has identified all of the issues affecting your service. This will cost a few $$$$ but it will be a pittance in the grander scheme of things.

Your tech should be able to specify the scope of necessary corrective work, and then test the system after the work is completed. If the problem is within Comcast's network (not likely, but possible), the tech should be able to ascertain that. In that event, use the tech as an intermediary between you and Comcast. Let them devise the remedy. Don't give up until the matter is resolved.