Why rested?
Was he over-exercised?
Why are the legs dodgy - can you post a pic?
Skycat talks about a genetic defect and I think this is spot-on (I work with biomechanics in humans, not dogs, apart from my own - just so we're all sure). Like in the recent Olympics, some people are made to run, and so are some dogs. Even in the right mix of dogs some are way better than others - it's in the blood, or more correctly, it's in the genes.
If the dog doesn't conform to your ideal you have two choices, get rid, or work with what you have.
From personal experience working with what you have