Maybe try exchanging some of your preacher curls for standing BB curls. Add this change though:
Get into normal curl position, but then squeeze your scapulae together and keep your elbows behind the coronal plane. Doing them this way it's all biceps working, and I've found it to work fairly well for emphasizing the outer head - which might help with your problem!
Give 'em a shot and see if you like them.