My first greyhound had a partially ruptured tendon which presented itself as a swelling on the tendon and a mild, persistent lameness. My own vets didn't know what it was even after xrays so I consulted a greyhound vet (one of the authors of Care of the Racing and Retired Greyhound) and he told me to leash-walk him for 6 months to allow the tendon to heal. It seemed like an awfully long time (and he was a dog who loved to run!), but the tendon did heal and we had no more problems with it even when he went back to running. Obviously I don't know if your dog has a similar problem, but it may be that there is a soft-tissue injury there which isn't getting sufficient time to heal before being aggravated by running again.