Do you have a neighbor that uses the same service? If they have similar issues, you can also help eliminate your house and wiring through that method. If they don't have similar issues, you've got someone who can maybe wifi you some decent internet.

Your best bet is to keep trying to escalate. At some point, you get to someone who understands the architecture of the service they're providing. The lower level techs all have great intentions, and many of them are very good - but they just don't have enough knowledge of the possible points of failure to really find the problem. Continuing to push consistently (sometimes for weeks - or even months) will get the problem escalated to someone who can fix it.