My DS did the same thing. I agree try and burp ...

My DS did the same thing. I agree try and burp him first to make sure that's not the prob. I found that it was just a faze he went through. I just leave him in position, let him have his scream and then when he was ready he'd feed. Not sure if he was doing this for any special attention, but i hoped by not reacting he'd get bored with it. It was frustrating because as they're getting more efficient you just want them to feed so they can have a play. He gave it up within a couple of weeks. Hope this helps