You do not mention if it is a front or rear leg, which might make a difference, but I assume your regular vet has already gone over most of the common causes of lameness.
The condition you describe sounds like it might be tendinitis. If the tendon that attaches to that toe is inflamed, and then becomes scarred and contracts, it can cause the toe to be lifted off the ground similar to that which you are describing.
I would ask my regular vet to investigate autoimmune diseases and things that might cause an inflammation of the tendon.
If the limping continues and he is not able to diagnose it, you should ask for a referral to a veterinary teaching hospital (university) in your area so that others can look at it and possibly give you a correct diagnosis.