On and off lameness is a dangerous thing and usually soft tissue related. It may be his knee. When you go to the Vet, mention it. I had a friend who weight pulls her dog. He had intermittent lameness. He did lure coursing that day and tore it through. Turns out it was damaged, but she received poor advise and kept working him. I would recommend less turns to none, especially if he is weighted or dragging. Swimming, straight pulls of light weight and walking figure eights, and stand and sit exercises. This and others are a good physical therapeutic regiment. Your Vet may be able to refer you to someone with the full regiment list. I would start this after a period of cage rest. Soft tissue takes an extremely long time to heal, and in the process you can put more pressure on the opposing leg, causing another injury. Good luck. Keep us posted on how it goes.