A specialist is probably a good idea at this point. Your vet can give you a referral to someone at a veterinary teaching hospital if there's one near you.
As Longblades pointed out to Lindsaygirl, this really is a very old thread--if you start your own with a descriptive title, you may get more input.
Good luck with your Lab!
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."