Dave, did they do an MRI of your lumbar spine? Does your back spasm or do your legs just go weak?

When my L5-S1 disk fragmented, I had very similar symptoms at first.
It turns out that the disk fragment was free floating in my spinal canal and when it would wedge up against my spinal cord, it would cause me excruciating pain that would drop my legs out from under me. Basically it would irriate my nerve roots and weaken my legs.
It did progressively get worse for me, the pain finally stayed and became unbearable as the fragment caused swelling in my spinal cord. I eventually had to be on high dose steriods for almost 2 weeks, then had to have a microdiskectomy to remove the fragment.

But what you describe is pretty much how exactly what I went through at first.