Have you checked to see if the rotors are out of balance? You mentioned that the brakes were from an unknown source. You might also try Dyna Beads inside the tires which will dynamically balance the wheels as you drive.
http://www.innovativebalancing.com/index.php
I have been running these for years and they work great. Plus you don't have to worry about periodic rebalancing and you don't have weights stuck on the wheels.