Back in 2006, after six long months of dealing with poor technical support from Comcast, I diagnosed the problem myself.
The issue I was having at the time was extreme internet speed loss, high packet loss and sometimes no connectivity for extensive lengths of time. After technicians came out to my house and did everything from change my modem to change the cable on the entire street, I discovered the real problem. Road construction crews had severed a number of cables along a stretch of highway near the house that was being widened, and those crews had buried the cables and pretended nothing happened. After spotting crews from other companies working on their lines, I mentioned to Comcast what I saw and believed to be the problem.
My service was running normally by the end of the business day. I also had six months worth of credit and was upgraded to a higher tier of broadband service.
It's not the first time I reported a problem to a company before the problem became apparent to that company. In 1998, while I was with Bellsouth for my internet service, I experienced troubles connecting to websites via web addresses, but no problems at all connecting via IP address. I phoned their technical service line to report the DNS server outage. The guy on the other end said, without missing a beat, that everything was fine. I challenged him to double check. Upon his own attempt to connect to websites, he simply told me that I was right.