What Causes Aching Pain In The Back And Numbness In Leg?

I have had an aching pain in my back for around 6 years. I also get periods of numbness in my right leg (feels like a dead leg), particularly around my hamstring. Every doctor I have spoken to has given me a different diagnosis, from muscle weakness to possible arthritis (which a blood test for came back negative). Pars defect has also been suggested. Any ideas?