As the xrays haven't shown up anything, why not take him to a chiropractor to see if that helps? Sunny has had intermittent lameness on his left front leg and the first vet had no idea what was causing it. The second (greyhound) vet we saw found a problem in his neck with a misalignment of one of the bones. He said this injury is very common in greyhounds, mostly after a fall or collision. Sunny had had a collision with his sister 10 weeks before we saw any symptoms. We tried painkillers and muscle-relaxants but they didn't cure the problem. The second vet said he could manipulate Sunny's neck under anaesthetic but I wanted to try a chiropractor first as I felt that was less invasive and risky. So, to cut a long story short, Sunny has seen the chiropractor 5 times now and is very much improved. He is going longer and longer between appointments, the last interval being almost 3 months. Might be worth trying before going down the MRI route?