Bad DSL connection...! HELP.?


OK so I have SBC(now A&T) as my ISP. how onnection has always been fine u untill about 3-4 months ago. I have done EVERYTHING. I called them mutlile times and got multiple answers.

1)Bad modem(we had it for seven years)
          >I bought a new actiontec modem. it still goes out!
2)Bad filter
         >took off the filter and plugged the phone into a diff line with a filter and the dsl directly
         >to the jack
3)Bad line
        >some of our hone jacks in the house are not wired right so they recieve no dialtone
        >the jack the phone and dsl are plugged into has a tone. only thing is  the jack its in has two
        >cat5 I cannot figure out there are for either.

The phone company says the line to our house is ok and no one else in our area is having the same prob. They can send out a tech to check the line to our house(no charge) but if he goes in the house and finds a problem(even if he doesnt fix it) we will be charged for the labor. I have googled this for hours and called tons of ppl. I dont ned an answer how to fix it(i would like one) but i really want to know what is causing it.