I found a very, very interesting discussion of 'cross canti brake issues on VeloNews via Leonard Zinn. The tenor of the article corroborates the fears that took hold of me on the DDRamble back in November, when frankly I thought I was going to die on a steep downhill while the cantis on the Crosscheck shuddered uncontrollably. Zinn's basic explanation deals with fork flex and cantis, implying that it's quite difficult to avoid, even for seasoned 'cross mechanics. His solution is to run a V-brake on the front and a canti on the back.
I don't think it helps the here-n-now, but it makes me feel a lot better than there are experienced bike folks out there with the same problems. The QB and Blueridge both have cantis, but with many miles in the BR never game me any problems, but I don't have enough experience with the QB. The LHT has direct-pulls and the Bleriot dual-pivot