Have you ever been checked to see if you have a disc problem? They asked me about this symptom when they discovered that I have a bulged and torn disc (fortunately, I don't have this problem).
It could be a disc problem causing a nerve problem that is causing both your back pain and your leg to go out. I think you should have it checked out. You should definitely call your doctor if you are having your legs collapse causing you to fall. That is not something to simply tolerate.
peripheral spondyloarthropy, HLA-B27+, iritis, naproxen 500 mg, 2x/day, trochanter bursitis, Joint Hypermobility Syndrome, migraines, SI joint dysfunction/hypermobility, DDD L4/L5 and L3/L4, straightening of cervical spine, diabetes, sleep apnea.