yes A high resistance fault on the line can give your symptoms but apparently they aren't easy to find and you may also struggle to get your ISP to despatch a BT engineer with your current level of service.

Thanks yarwell, at least you've provided a hint as to what the problem could be. A little glimmer of hope which I will hold on to and pester my ISP about.

If anyone has any other suggestions feel free to comment and Yarwell if something else crops into your head do tell.