I saw a show on Mystery Diagnosis about a lady with a balance problem. She had a vitamin B deficiency. A rhumatologist may be able to help you.
Piriformis Syndrome should not cause a loss of balance. I had it once myself. Physical therapy was the most helpful by far - but there are more controversial treatments like botox therapy to paralize the muscle, or a trigger point injection with bupivicane or lidocaine. The Loss of Balance would be due to another problem. Virus maybe?