I've been struggling with my DSL connection for a while now. Over the past week it became intolerable, and I dug in to investigate this weekend.
I had been experiencing intermittent outages, which I wrote off as bad service (Ameritch and I have had issues in the past*). Over the past week I started experiencing significant packet loss (>30% on pings to the ISP's DNS server), so I knew it was getting worse.
Doing some basic troubleshooting, I thought I heard 'noise' on the line, so I replaced the DSL filter I setup. No luck.
I gave up and called SBC (Ameritech). They immediately acknowledged that something wasn't right, and referred me to their troubleshooting center. The next morning I got an automated call that I needed to call them to discuss. After some debugging on the phone, they decided that it was probably my DSL Modem. Since it was > 5 years old they thought it was probably starting to flake. I went out and bought a new one.
The new modems actually have a nice web interface so you can see what is going on. Unfortunately, it told me that my downstream connection was ~ 500 Kbps, while my upstream was 640 Kbps. Yes, this is WRONG. The downstream should have been 6 Mbps, or ~6,000Kbps. I called them back, and they agreed it was still wrong, and dispatched a technician, with the warning that there may be a charge if the problem is in my internal wiring.
I decided to verify that the issue was external, so I took my laptop, DSL Modem, and some wires outside and plugged them directly into the telephone box on the outside of my house. To my surprise it connected at full speed, indicating that the issue really was internal.
I tried some simple debugging (unplugging devices, etc.), but eventually just ran a fresh line from the inside junction box to my DSL modem. Success!
I'm a little annoyed I couldn't find the problem, but there must be some type of interference on the wiring inside the house. However, since it is all effectively hard wired together, it is nearly impossible (well, given a reasonable amount of effort) to track down. So, I'll have to settle for good enough.
* 'Back in the day' I had a frustrating experience with Ameritch (or whoever they were back then). There was noise on our line, causing my modem all sorts of problems. What really annoyed me was when they told me they only supported speeds up to 9600 bps, and if I was using a faster modem I was simply out of luck. It took a while but I convinced them that the issue was bad enough I could hear with my own ears. They ended up dispatching a truck and found that there was 'water in the line' on their end.