I'll check the the H limit screw on both der.'s again. I suspect, though, that If it were an issue with the H screw at the front, it would fall off when shifting to the large chainwheel no matter what gear I'm in at the back. Same for the back H screw, except it would fall off no matter what chainwheel I was using.

I've also readjusted both der.'s from scratch again. Same problem.

I'll measure my chainline and post when I get back from my ride.