SimonGeorgeBrown: I don't even want to remember the headaches that WorldxChange caused me. I always found their customer service rude and pointless. Nothing ever got done and I had the same issues as you, and then some. I recommend dumping them as soon as possibly.
Without defending WxC in any way I'm almost willing to put money on the issue being a wiring issue within the OP's home. WxC have suggested this as the cause and have quite clearly tried numerous other possible solutions.
By far the vast majority of DSL speed and connection related issues are within the home, particularly with wiring, and there is nothing that the ISP can do in this case except for making this clear to the account holder, most of whom don't want to believe this or don't want to pay to fix the issue.
IMHO there should be NO self install options for DSL. Without a master DSL filter you will get a substandard connection in the vast majority of cases, however most people don't want to pay money to get a professional job done the first time around, and suffer from the consequences, typically blaming their ISP for the issue. I'd take a stab right now and say there are hundreds of posts on here over the years where somebody ranting about their DSL issues and laying all the blame on the ISP and or Telecom when the issue has been with their home wiring.