In re-reading this thread you don't mention what kind of telephone wire you have in your house?
Like Leibold has suggested, using Cat 3 or Cat 5 twisted pair cable will definitely give the best results.
Years ago I discovered my place is wired with the typical the Radio Shack type two parallel pair telephone wire (non-twisted) and every time I received a call on my second line, my ADSL suffered or would disconnect due to interference.
I solved the problem by running a CAT5 drop line from the NID directly to my computer, bypassing the in wall telephone wire completely. Is this a possibility for you?
If so, educate yourself about proper CAT5 installation procedures such as avoiding electrical boxes and rounding the diameter of turns like a Coke-Cola can
Also, I can confirm from experience switching from a Covad installed ADSL splitter to a Sonic installed ADSL2+ splitter increased my speeds significantly.