Yes, I would agree. 400 feet is a long run for untwisted "quad" cable (category 0!).
If there is a second line on the quad (e.g. the yellow black wires) then you'll have problems for sure.
You'll want to replace it with Cat. 3 or better.
That said, first you may want to try connecting the adsl modem directly to the network interface and see if it works better.
While you are at it, get a whole house filter. Run the "phone" output on the blue/white pair to the reast of the house. Run the "DSL" output on the orange/white pair and connect that only to the jack where the DSL modem will live.
A slightly simpler alternative to repalcing all 400 feet of the quad is to connect the exisiting quad to the "phone" output of the filter and then make a new run, using cat. 3, from the "DSL" output of the filter to a new jack for the modem.
If you watch ebay, you should be able to get a decent price on a whole house splitter. I got a good deal on a couple of Siecor units last year.
Good luck