Ive got a simple solution for the preacher curls...dont do 'em! Its not as if preacher curls are a main move like deadlift and bench, its a little ol' biceps exercise and theres hundreds of others to choose from. Why stick with one that hurts you???
As for the other question, I dont know, changing grips is all i can suggest.
I'd hate to abandon preacher curls because I DO find them an extremely effective bicep mass builder particularly with a barbell. You may want to vary the seat height adjustment on the apparatus. This sounds irrevelant but depending on your body's position is where the stress falls on the biceps.
Also try icing and heat pads which always work well. 15 minutes of each alternating. Biceps are smaller than back so naturally they fail first and as the other poster mentioned you must focus on contracting your back and not pulling with your arms.