I tried the string test on the frame and it looks very straight. I think the problem is with the fork. Its subtle but it looks like one side has a little less rake than the other. A buddie (we did a quick 40k ride, what weather!) thought the front wheel was leaning very slightly to the right.

I think I'll take the fork off and lay it on the marble counter with a large piece of lined paper underneath. I should be able to take some measurements that way. I can cobble up a tubing bender fixture in the woodshop.

David