I have a dsl line from Qwest. I use to have many of the problems you are describing. I use to call them every day until I got a tech to come out and test the line.
The problem was the connections between my house and their office. It took me months to get it fixed.
Everything was fine after that until one day I started having the same problem. This took another month. Seems this time somebody switched the line at the local box and I was back to the same problem.
Have someone check all the connections in your house first. If they are ok then the problem is them. They will want you to pay for that but at least you might get someone to come to the house. I find the people are much more reasonable than the company.
While I was going through all that the one thing I found was that calling late at night worked much better than calling during the day.